Eur Spine J (2015) 24:80–87 DOI 10.1007/s00586-014-3485-6
ORIGINAL ARTICLE
‘Pseudofacets’ or ‘supernumerary facets’ in congenital atlanto-axial dislocation: boon or bane? Pravin Salunke • Sameer Futane • Manish Sharma Sushant Sahoo • Udaybhanu kovilapu • N. K. Khandelwal
•
Received: 30 January 2014 / Revised: 20 July 2014 / Accepted: 21 July 2014 / Published online: 30 July 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Certain abnormal contact points, appearing like additional joints (pseudofacets) were observed between atlas and axis in a subset of patients with congenital atlantoaxial dislocation (CAAD). The origin, function and bearing on management of such pseudofacets remain largely undetermined. The object is to study ‘pseudofacets’or ‘accessory joints’ in patients with CAAD and to analyze the possible genesis, role and bearing of these on surgery and fusion rates. Materials and methods 35 patients with CAAD were analyzed. Reconstructed images of CT craniovertebral junction passing through these pseudo and true facets were studied. A novel method was devised to measure the faceto-isthmic angle of axis, both in patients with CAAD and normal subjects. Operative details and fusion rates were studied in patients with pseudofacets and compared with those without it.
P. Salunke (&) S. Futane M. Sharma S. Sahoo Department of Neurosurgery, PGIMER, Sector 12, Chandigarh 160012, India e-mail: [email protected] S. Futane e-mail: [email protected]
Results Eight out of 35 patients (6 Irreducible CAAD and 2 with RCAAD) had pseudofacets. These are seen posterior to the true facets and resemble partially formed joints. The C2 facet was acutely bent over its isthmus in these patients. The direction of these pseudofacets appeared to counter the abnormal mobility at C1–2 true facets. Intraoperatively, they posed a visual hindrance to reach up to true facets for placement of spacers and lateral mass screws, requiring extensive drilling. At the same time, they did help in distraction and increased the surface for fusion between C1 and C2 in cases where sublaminar wiring alone was used. Fusion rates were 100 % in patients with pseudofacets. Conclusions Pseudofacets may be a result of genetic aberration and nature’s mechanism to restrict abnormal C1–2 mobility in CAAD by imparting some stability. Their presence hinders the visualization making it difficult to reach upto the true facets, thus a bane. They may require extensive drilling when direct posterior approach is used, thereby disrupting the natural restrictive mechanism. However, the flattened surfaces provide an increased area for postoperative bony fusion between C1 and 2, making their presence a ‘boon’. Keywords Pseudofacets Congential atlanto-axial dislocation Facet joint Supernumerary joints Accessory joints Surgical outcome
M. Sharma e-mail: [email protected] S. Sahoo e-mail: [email protected] U. kovilapu N. K. Khandelwal Department of Radiodiagnosis, PGIMER, Chandigarh, India e-mail: [email protected] N. K. Khandelwal e-mail: [email protected]
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Introduction The congenital atlanto-axial dislocation (CAAD) is likely to be a dynamic process; progressing with time [1]. There would be certain naturally occurring mechanisms, albeit unknown, restricting or slowing its progress. We observed certain abnormal areas of contact between C1 and C2 in
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We evaluated 35 patients with CAAD operated in the last 3 years. Of these 23 patients had irreducible (Ir) and 12 had reducible (R)CAAD. ‘Pseudofacets’ were defined as abnormal contact between C1 and C2, with flattened surfaces, usually seen just posterior to the isthmus of C2 and inferior surface of C1 posterior arch laterally. The imaging of patients with pseudofacets was studied retrospectively, except for the cases operated in last 1 year where they were picked up before surgery. Parasagittal sections and axial
cuts passing through the facets were obtained. The parasagittal section showing C1–2 facet and the portion of C2 lamina just posterior to the facet was chosen. The C2 lamina appeared to be more angulated over facets in patients with pseudofacets. To quantify this angulation, we devised a novel method: C2 faceto-isthmic angle that was defined as the angle between the line drawn along the superior surface of C2 facet and the line along the superior surface of the pars interarticularis (Fig. 1). Furthermore, the orientation of the pseudofacet was along the pars. (Fig. 1). Thus, the angle also gave an idea about the orientation of the pseudofacets in sagittal plane. It was measured for both sides. The length of the pseudofacet was measured in this section. These angles were compared with ten normal CT of cervical spine (obtained as a part of polytrauma protocol). Besides this, the C1 inferior facetal angles were calculated in all patients [1]. Axial CT images passing through the C1 arch were obtained. Telescoping of C2 or central dislocation was looked for. This was defined as the presence of C2 lateral masses, the part of lamina just posterior to the facets and the body of C2 in the same section showing the C1 arch. Apart from the telescoping, the pseudofacets could be appreciated in the axial sections (Fig. 2c arrow). The width was measured in the axial
Fig. 1 a–c Showing diagrammatic representation of atlanto-axial facet joint architecture varying from normal to reducible dislocation and to a fixed one with formation of ‘‘pseudofacets’’, respectively. The vectors of forces acting across facet joints in these conditions are shown by arrows of increasing thickness suggesting incrementing magnitude of force. With increasing acuteness of C2 faceto isthmic
angle, the portion just posterior to isthmus touches C1 arch increasing the surface area thereby dissipating the pressure. The dotted red circle represents C1–2 ‘‘pseudofacets’’. d–f Shows the C2 faceto-isthmic angle as seen in normal (obtuse), reducible AAD (nearing 900) and irreducible AAD with pseudofacets (acute) in parasagittal images of non contrast CT scan
some cases of CAAD that possibly prevents progression of CAAD. These were usually seen just posterior to the isthmus of C2 and the inferior surface of C1 posterior arch laterally. These abnormal contact points resembled true facets to some extent, but lacked all features of a true joint and hence were labeled as ‘pseudofacets’ or ‘supernumerary facets’. The purpose of this study was to retrospectively analyze these ‘pseudofacets’ (abnormal contacts between C1–2) in cases of CAAD. Based on the observations made, we have proposed the possible genesis and their bearing on surgery and outcome.
Materials and methods
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Fig. 2 Non contrast CT scans showing a patient with severe IrCAAD in mid-sagittal section. b, c Showing same patient’s parasagittal and axial section, respectively. The arrow showing the abnormal areas of C1–C2 contact posterior to true facets joints. These ‘joint-like’ areas
are being labelled as ‘‘Pseudofacets’’. d–f Showing different patients with similar ‘‘pseudofacets’’ (arrows) with acute C2 faceto-isthmic angles. The arrowheads represent the true facets
section showing maximum pseudofacet. The exact dimensions of pseudofacets were difficult to measure as many of them were reconstructed images from thick slices of base CT. CT angio was obtained in the cases operated in the last 1.5 years to look for anomalous vertebral artery (VA). Operative videos and notes were reviewed at to specially study these pseudofacets. In the first 1.5 years patients with IrCAAD had undergone transoral decompression and posterior fusion. However, in the last 1.5 years we have switched over to direct posterior open reduction and fixation for these patients. Fusion was looked for in these patients at follow-up. Fusion was defined as the single cortical margin seen between C1 and C2.
within C1 (basilar invagination). Two of them also had multiple level fusions of subaxial spine. Such bony abnormalities were absent in the two patients having of RCAAD with pseudofacets (Table 1). The mean C2 faceto-isthmic angle, inferior sagittal C1 facetal angle in all patients of CAAD with pseudofacets, CAAD without pseudofacets and normal subjects are compared in Table 2. The mean C2 faceto-isthmic angle in patients of IrCAAD with pseudofacets was acute as compared to patients of IrCAAD without pseudofacets and normal subjects. Additionally, more acute the sagittal inferior C1 facetal angle, more acute was the C2 faceto-isthmic angle (Table 1).The orientation of pseudofacets in the sagittal plane was cranio-caudal anteroposteriorly, as opposed to the caudo-cranial antero-posterior sagittal inferior C1 facet or superior border of C2 facet. This orientation of the pseudofacet appeared to counter the mobility of C1–2 true facet at least in the sagittal plane. The faceto-isthmic angles in the two patients having RCAAD with pseudofacets were relatively acute as compared to those without it and normal subjects. In one of these two patients, the pseudofacets were compressing the cord. The pseudofacets appeared to be larger in these two patients with RCAAD as compared to the pseudofacets in patients with IrCAAD.
Results Out of 35 patients, pseudo facets were observed in eight patients with CAAD (Table 1). The age varied from 3 to 25 years. Pseudofacets were seen in six patients with IrCAAD (26 %) and in two patients of RCAAD (16 %). The approximate dimensions of these pseudofacets are mentioned in Table 1. All the six patients of IrCAAD with pseudofacets had occipitalised atlas, C2-3 fusion and telescoping of C2
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M/15 F/3
M/12
F/11
M/25
M/18
M/24
M/21
1 2
3
4
5
6
7
8
RCAAD (no central dislocation)
RCAAD (no central dislocation)
IrCAAD (central?ant-post)
IrCAAD (central?ant-post) with situs inversus and dextrocardia
IrCAAD (central?ant-post)
IrCAAD (central?ant-post)
IrCAAD (central?ant-post) IrCAAD (central?ant-post)
Diagnosis
–
?
?
?
?
?
? ?
Presence of occipitalised atlas and C2-3 fusion
PF with C1 lateral mass and C2 pars interarticularis screw
PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and p with SLW
TOD?OC2 fusion (SLW)
TOD?OC2 fusion (SLW) TOD?OC2 fusion (SLW)
Surgery
162
152
120
146
135
145
135 110
C1 sag inferior facetal angle
86
70
62
44
61
100
72 45
C2 facetoisthmic angle
16
13
8
13
10
13
8 9
12
16
6
12
8
3
9 6
11
10
10
10
12
11
11 8
9
14
8
8
11
2
10 6
Left
Right
Right
Left
Width of pseudofacet (mm)
Length of pseudofacet (mm)
10 months
12 months
10 months
1 year
1 year
1.5 years
3 years 2 years
Follow-up
Y
Y
Y
Y
Y
Y
Y Y
C1–C2 bony fusion
IrCAAD irreducible congenital atlanto-axial dislocation, RCAAD reducible congenital atlanto-axial dislocation, central dislocation basilar invagination, TOD transoral decompression, OR open reduction, PF posterior fusion, SLW sublaminar wire
The C1 inf sagittal facetal angles and C2 faceto-isthmic angles are the average of right and left
Age/ sex
No
Table 1 Details of patients of CAAD with C1–2 ‘pseudofacets’
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Table 2 Comparison of the sagittal inferior C1 facetal angle and C2 faceto-isthmic angles in normal subjects and patients with CAAD (with and without pseudofacets) Normal [(10) Sagittal inferior C1 facetal angle
172.6 ± 4.5 (160–176)
C2 faceto-isthmic angle
135.2 ± 5 (130–145)
RCAAD without pseudofacets (10) 165.2 ± 7 (160–170) 105.8 ± 10.9 (90–120)
RCAAD with pseudofacets (2)
IrCAAD without pseudofacets (17)
IrCAAD with pseudofacets (6)
157 (152–162)
140.3 ± 18.4 (122–175)
134.4 ± 14.6 (110–150)
104.4 ± 14.2 (75–125)
64.8 ± 18.9 (40–100)
78 (70–88)
Number of patients in each subset is mentioned in parenthesis
Fig. 3 A patient of IrCAAD with pseudofacets. The upper row shows reconstructed 3DCT showing pseudofacets (marked by asterisk) and true facets (marked by arrows). The middle row shows a 2DCT image showing true (asterisk) and pseudofacets (arrow) in the same patient. The lower row shows intraoperative image showing right and left
pseudofacets (marked by asterisk) and the distracted true facets (marked arrow). The pseudofacet on left side was bulbous and smooth. This was drilled and a part was sent for histopathology that showed osseous tissue flanked by hyaline cartilage but lacked synovial membrane
There was no statistical difference in atlanto-dental interval of patients of CAAD with pseudofacets and those without it. Figure 2 depicts the psuedofacets in various patients. There was anomalous VA’s in one of the patients with IrCAAD and pseudofacets. Radiologically and intraoperatively the pseudofacets were bilateral, though asymmetrical (right and left) in all patients. The surfaces in five cases (2 with RCAAD and 3
with IrCAAD) appeared smooth with presence of cartilage (Figs. 3, 4). The presence of hyaline cartilage was proved by histopathology in two of these cases in whom the tissue was sent for histopathology. However, these lacked synovial membrane or capsule. The remaining three cases had rough surfaces and were almost in apposition. In these patients they appeared to be apposing flattened bony outgrowths. In one patient it appeared fused on the posterior
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Fig. 4 a Intraoperative images showing C1 and C2 ‘‘pseudofacets’’ (PF) with C2 spinous process (SP) in patient with RCAAD. Black arrowheads showing the C2 lamina hidden behind the gauze. b Showing C2 nerve root (C2 N) lying anterior to ‘‘pseudofacets’’ helping to differentiate true versus false facets, intraoperatively. c Shows completely opened up pseudofacets with dissector in true facets. The glistening smooth ‘‘joint-like’’ surface of C2-PF is
appreciable. d After cutting the C2 N and drilling off C1-PF the joint cleft of C1–C2 facet joint. e, f Extension-flexion CVJ X-rays of the same patient. The solid white arrows mark the pseudofacets and black thin arrows mark the true facets. g Excised specimen of pseudofacets of another patient with RCAAD. Note the smooth glistening surface (solid white arrows), resembling articulating surfaces of true synovial joint
Fig. 5 The upper row shows CT scan of a patient with severe AAD. a Preoperative image. b, c Showing post-operative images showing requirement of excessive drilling due to ‘‘pseudofacets’’ to open and reduce true facets at the cost of decreased bony purchase for C2 screws. Notice the drilled flat C1–C2 true facets with interposed bone
grafts. The complete reduction and stabilisation is seen in (d). The lower row shows another patient with ‘fixed AAD with pseudofacets’ (e) wherein ‘‘pseudofacets’’ itself are used for distraction and placement of bone graft (shown by arrow in f) and satisfactory reduction is achieved (g)
edge and could be distracted only after drilling. The C2 nerve roots were ventral to these pseudofacets and separated them from the true facets.
Of the six patients with IrCAAD with pseudofacets, transoral decompression with posterior fusion was performed in three patients. In these, posterior fusion was
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achieved with sublaminar double atlas cable. The pseudo facets were not drilled extensively as the true facets were not opened in these cases. The surfaces of C2 and occiput were decorticated to aid fusion. Direct posterior reduction and fusion were used in the remaining three patients with IrCAAD and pseudofacets. Of these three patients with IrCAAD, the pseudofacets were drilled extensively in two, to reach up to the true facets. The drilling was required because the pseudofacets came in the surgeon’s line of vision of true facets. This drilling reduced the purchase for C2 translaminar screws (Fig. 5 upper row). In the other patient (age 11), distracting these pseudofacets itself opened the true C1–2 facets and reduced the AAD to a large extent. Following this bone graft was placed between the both true and pseudofacets after drilling their surfaces to augment fusion with sublaminar wires (Fig. 5 lower row). In the patient with anomalous VA, the artery was safeguarded by dissecting it and retracting it during drilling of joint. In this case the artery was seen anterior to C2 nerve root but posterior to C1 facet. The 2 patients of RCAAD with pseudofacets underwent posterior fusion (lateral mass screws) in reduced position. In one of these two patients, the pseudofacets were causing cord compression and had to be excised. Histopathological evaluation of these pseudofacets was obtained in two cases (one with RCAAD and other with IrCAAD). It revealed osseous tissue flanked by regular hyaline cartilage but lacked synovial membrane or capsule [2]. Though the osseous tissue and the regular hyaline cartilage resembled true joint, the lack of synovium suggested it to be dysmorphic or partially formed joint. Post-operative bony fusion was seen in all the eight patients with pseudofacets. Whereas, post operative bony fusion was seen in 18 patients of the 27 with CAAD without pseudofacets. Nine patients, in whom bony fusion could not be appreciated, had no mobility on flexion extension (except one that was reoperated).
Discussion Accessory or additional joints at the CVJ, though rare have been described in patients with RCAAD [2, 3]. Histopathology had proved them to be different from true synovial joints (lack of synovial membrane) and hence we labeled them as ‘pseudofacets’. The presence of such pseudofacets’ has not been described in IrCAAD previously. They seem to have a role in imparting stability. Radiological, intraoperative and histopathological observations were analyzed in patients of CAAD with pseudofacets. Based on these observations, we have listed the possible reasons for their development and role. Besides this, their bearing on surgery and influence on outcome has been discussed.
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The C1–2 pseudofacets were more often observed in patients with acute bending of C2 lamina over facets (Fig. 1). This kind of angulation (acute C2 faceto-isthmic angle) is unlikely to increase with age and is more likely to be congenital. Additionally, it was seen in young patients (age ranged from 3 to 25 years). Thus, the presence of pseudofacets may be congenital. Embryologically, the genetic aberration of HOX genes, could have initiated the process of interzone formation at the future sites of pseudofacets. The abnormal movement at C1–2 joints would have lead to cavitation in this zone which is a future harbinger of a joint cavity [3]. The ill-sustained process (for the lack of adequate signal) may have ended up in an incomplete joint formation. The fate of the pseudofacets is decided by the embryonic time frame where the process ends. Early arrest would lead only a bony dysmorphic facet, whereas a late arrest would lead to dysmorphic facet flanked by articular cartilage without a synovial membrane and capsule [2]. Such osseous tissue with hyaline cartilage lacking synovial membrane was seen on histopathology in two of our patients. Theoretically, it is possible to have a well-formed ‘true facet joint’ with capsule and synovium at an abnormal location if process of genetic aberration is completed with proper inductive signals. The ‘genetic abberation’ theory, does not explain the variability of age at presentation of these patients. The increase in surface area of pseudofacets or even the formation of pseudofacets secondary to the CAAD cannot be ruled out. The orientation of facetal planes of atlas (C1) and axis (C2) appear to be an important factor in the antero-posterior and vertical dislocation in the Cranio-vertebral junction abnormality. The telescoping of C2 within C1 as well as the antero-posterior dislocation, possibly progresses with time (dynamic process) [1]. This continues till the portion of the C1 arch touches the C2 lamina just posterior to the isthmus, where the pseudofacets may already be existing. Furthermore, such contacts would occur earlier in patients with acute faceto-isthmic angles. This contact between C1 and C2 flattens and increases the surface area in an attempt to reduce the instability (Fig. 1c). These surfaces may fuse partially leading to C1–2 autofusion. This is akin to osteophytes in cervical spondylosis [4]. Occipitalised atlas along with C2-3 fusion increases the stress on the already malformed C1 and C2 facets, thereby augmenting the surface areas of pseudofacets to reduce the instability. Bony outgrowths are often seen at craniovertebral junction. However, these are never covered with cartilage. This increase in the surface area of pseudofacets itself may cause cord compression as was seen in one of our patients [2]. The above theory fails to explain the presence of pseudofacets in RCAAD (normal faceto-isthmic angles), though it was seen only in 2 out of 12 patients with
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RCAAD. Additionally, both patients with RCAAD had pseudofacets that simulated true facets and had larger sizes than their counterparts in IrCAAD. So it is likely to be a result of genetic aberration that is accentuated by the existing instability. Apart from this, we observed bending of C1 arch over the facets or incurving of the occiput. Unfortunately we could not define fixed points to measure them in normal subjects making it difficult to compare objectively. Whatever be the etiology, the number of contact points between C1 and C2 increases to four (two at the true facets and two at pseudofacets) thereby adding to the stability. Apart from this, the orientation and direction of pseudofacet is such that at least the antero-posterior vector at the inclined true facets is countered (Fig. 1b, c). The vertical vector remains, continuing the central dislocation. In short, ‘pseudofacets’ appear to impart stability and may be natures attempt to counter abnormal C1–2 mobility. Pseudofacets have a bearing on the management of atlanto-axial dislocations. These pseudofacets, can be distracted helping in intraoperative reduction of CAAD. These ‘pseudofacets’, by the sheer virtue of their large surface areas may actually help occipito-C2 or C1–2 bony fusion, provided they are decorticated. Even with sublaminar wires good outcome can be achieved. This makes the presence of ‘pseudofacets’ a boon. The management of IrCAAD has changed significantly in last decade. The pendulum has swung from transoral decompression to direct posterior reduction and fusion [5– 7]. Similarly, various treatment strategies for severe C1–2 subluxation have been discussed by Bach et al. [8]. Further discussion by Goel, suggests tackling the C1–2 joints even in such severe subluxations without removal of posterior arch [8, 9]. The direct posterior approach requires opening up, drilling of the true facets and placement of spacers. The anomalous VA needs to be safeguarded while the joints are being comprehensively drilled [10]. The pseudofacets come in the surgeon’s line of vision of true facets, making drilling of C1–2 true facets and placement of spacers and screws difficult. This makes their presence a ‘bane’. To reach the C1–2 facets, these pseudo-joints need to be drilled as simple distraction opens them up only transiently. The C2 nerve root is consistently found between pseudo and true facets. The inferior pseudofacet is formed by the portion of C2 lamina just posterior to the isthmus. The placement of C2 screws becomes difficult if a portion of this C2 pseudofacet is drilled as it reduces the available bony purchase. So it is preferable to drill the C1 pseudofacet. Apart from this, drilling of these pseudofacets may disrupt the nature’s compensation for instability. So this step, in a way may actually be counter-productive. The final C1–2 fusion needs to be strong.
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The postoperative bony fusion was seen only in 66 % of the patients of CAAD without pseudofacets. Whereas, postoperative bony fusion was seen in all patients of CAAD with pseudofacets. The reasons for this could be multiple. The technique of fusion has shifted to extensive decortication of facets from simple sublaminar wiring. The drilling is even more extensive in patients with pseudofacets. Secondly, the surface area of contact is more in patients with pseudofacets. Small number of patients is the limitation of this retrospective study. Additionally, the theory for formation of pseudofacets is based on a conjecture that needs to be proved by further studies. Nevertheless, it attempts to bring nature’s compensatory mechanism in CAAD to notice and that these ‘pseudofacets’ should be kept in mind while operating as they influence the surgical management and outcome. Conflict of interest
None.
Ethical standards Retrospective study of radiology and operative details of routinely operated cases of atlantoaxial dislocation.
References 1. Salunke P, Sharma M, Sodhi HB, Mukherjee KK, Khandelwal NK (2011) Congenital atlantoaxial dislocation: a dynamic process and role of facets in irreducibility. J Neurosurg Spine 15(6):678–685 2. Salunke P, Futane S, Vaiphei K (2013) Posterior bilateral supernumerary atlanto-axial facets: true or false joint? Spine (Phila Pa 1976) 3. Riesenburger RI, Klimo P Jr, Yao KC (2011) A fourth atlantoaxial joint: an initial description. Spine J 11(4):e6–e9 4. Shedid D, Benzel EC (2007) Cervical spondylosis anatomy: pathophysiology and biomechanics. Neurosurgery 60:S1-7–S1-13 5. Goel A, Kulkarni AG, Sharma P (2005) Reduction of fixed atlantoaxial dislocation in 24 cases: technical note. J Neurosurg Spine 2(4):505–509 6. Harms J, Melcher RP (2001) Posterior C1–C2 fusion with polyaxial screw and rod fixation. Spine (Phila Pa 1976) 26(22):2467–2471 7. Jian FZ, Chen Z, Wrede KH, Samii M, Ling F (2010) Direct posterior reduction and fixation for the treatment of basilar invagination with atlantoaxial dislocation. Neurosurgery 66(4):678–687 8. Bach CM, Arbab D, Thaler M (2013) Treatment strategies for severe C1–C2 luxation due to congenital os odontoideum causing tetraplegia. Eur Spine J 22(1):29–35 9. Goel A (2013) Expert’s comment concerning grand rounds case entitled ‘‘treatment strategies for severe C1–C2 luxation due to congenital os odontoideum causing tetraplegia’’ (by C. M. Bach, D. Arbab and M. Thaler. Eur Spine J 22(1):36–38. doi:10.1007/ s00586-012-2329-5 10. Salunke P, Futane S, Sahoo SK, Ghuman MS, Khandelwal N (2014) Operative nuances to safeguard anomalous vertebral artery without compromising the surgery for congenital atlantoaxial dislocation: untying a tough knot between vessel and bone. J Neurosurg Spine 20(1):5–10
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ORIGINAL ARTICLE
‘Pseudofacets’ or ‘supernumerary facets’ in congenital atlanto-axial dislocation: boon or bane? Pravin Salunke • Sameer Futane • Manish Sharma Sushant Sahoo • Udaybhanu kovilapu • N. K. Khandelwal
•
Received: 30 January 2014 / Revised: 20 July 2014 / Accepted: 21 July 2014 / Published online: 30 July 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Certain abnormal contact points, appearing like additional joints (pseudofacets) were observed between atlas and axis in a subset of patients with congenital atlantoaxial dislocation (CAAD). The origin, function and bearing on management of such pseudofacets remain largely undetermined. The object is to study ‘pseudofacets’or ‘accessory joints’ in patients with CAAD and to analyze the possible genesis, role and bearing of these on surgery and fusion rates. Materials and methods 35 patients with CAAD were analyzed. Reconstructed images of CT craniovertebral junction passing through these pseudo and true facets were studied. A novel method was devised to measure the faceto-isthmic angle of axis, both in patients with CAAD and normal subjects. Operative details and fusion rates were studied in patients with pseudofacets and compared with those without it.
P. Salunke (&) S. Futane M. Sharma S. Sahoo Department of Neurosurgery, PGIMER, Sector 12, Chandigarh 160012, India e-mail: [email protected] S. Futane e-mail: [email protected]
Results Eight out of 35 patients (6 Irreducible CAAD and 2 with RCAAD) had pseudofacets. These are seen posterior to the true facets and resemble partially formed joints. The C2 facet was acutely bent over its isthmus in these patients. The direction of these pseudofacets appeared to counter the abnormal mobility at C1–2 true facets. Intraoperatively, they posed a visual hindrance to reach up to true facets for placement of spacers and lateral mass screws, requiring extensive drilling. At the same time, they did help in distraction and increased the surface for fusion between C1 and C2 in cases where sublaminar wiring alone was used. Fusion rates were 100 % in patients with pseudofacets. Conclusions Pseudofacets may be a result of genetic aberration and nature’s mechanism to restrict abnormal C1–2 mobility in CAAD by imparting some stability. Their presence hinders the visualization making it difficult to reach upto the true facets, thus a bane. They may require extensive drilling when direct posterior approach is used, thereby disrupting the natural restrictive mechanism. However, the flattened surfaces provide an increased area for postoperative bony fusion between C1 and 2, making their presence a ‘boon’. Keywords Pseudofacets Congential atlanto-axial dislocation Facet joint Supernumerary joints Accessory joints Surgical outcome
M. Sharma e-mail: [email protected] S. Sahoo e-mail: [email protected] U. kovilapu N. K. Khandelwal Department of Radiodiagnosis, PGIMER, Chandigarh, India e-mail: [email protected] N. K. Khandelwal e-mail: [email protected]
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Introduction The congenital atlanto-axial dislocation (CAAD) is likely to be a dynamic process; progressing with time [1]. There would be certain naturally occurring mechanisms, albeit unknown, restricting or slowing its progress. We observed certain abnormal areas of contact between C1 and C2 in
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We evaluated 35 patients with CAAD operated in the last 3 years. Of these 23 patients had irreducible (Ir) and 12 had reducible (R)CAAD. ‘Pseudofacets’ were defined as abnormal contact between C1 and C2, with flattened surfaces, usually seen just posterior to the isthmus of C2 and inferior surface of C1 posterior arch laterally. The imaging of patients with pseudofacets was studied retrospectively, except for the cases operated in last 1 year where they were picked up before surgery. Parasagittal sections and axial
cuts passing through the facets were obtained. The parasagittal section showing C1–2 facet and the portion of C2 lamina just posterior to the facet was chosen. The C2 lamina appeared to be more angulated over facets in patients with pseudofacets. To quantify this angulation, we devised a novel method: C2 faceto-isthmic angle that was defined as the angle between the line drawn along the superior surface of C2 facet and the line along the superior surface of the pars interarticularis (Fig. 1). Furthermore, the orientation of the pseudofacet was along the pars. (Fig. 1). Thus, the angle also gave an idea about the orientation of the pseudofacets in sagittal plane. It was measured for both sides. The length of the pseudofacet was measured in this section. These angles were compared with ten normal CT of cervical spine (obtained as a part of polytrauma protocol). Besides this, the C1 inferior facetal angles were calculated in all patients [1]. Axial CT images passing through the C1 arch were obtained. Telescoping of C2 or central dislocation was looked for. This was defined as the presence of C2 lateral masses, the part of lamina just posterior to the facets and the body of C2 in the same section showing the C1 arch. Apart from the telescoping, the pseudofacets could be appreciated in the axial sections (Fig. 2c arrow). The width was measured in the axial
Fig. 1 a–c Showing diagrammatic representation of atlanto-axial facet joint architecture varying from normal to reducible dislocation and to a fixed one with formation of ‘‘pseudofacets’’, respectively. The vectors of forces acting across facet joints in these conditions are shown by arrows of increasing thickness suggesting incrementing magnitude of force. With increasing acuteness of C2 faceto isthmic
angle, the portion just posterior to isthmus touches C1 arch increasing the surface area thereby dissipating the pressure. The dotted red circle represents C1–2 ‘‘pseudofacets’’. d–f Shows the C2 faceto-isthmic angle as seen in normal (obtuse), reducible AAD (nearing 900) and irreducible AAD with pseudofacets (acute) in parasagittal images of non contrast CT scan
some cases of CAAD that possibly prevents progression of CAAD. These were usually seen just posterior to the isthmus of C2 and the inferior surface of C1 posterior arch laterally. These abnormal contact points resembled true facets to some extent, but lacked all features of a true joint and hence were labeled as ‘pseudofacets’ or ‘supernumerary facets’. The purpose of this study was to retrospectively analyze these ‘pseudofacets’ (abnormal contacts between C1–2) in cases of CAAD. Based on the observations made, we have proposed the possible genesis and their bearing on surgery and outcome.
Materials and methods
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Fig. 2 Non contrast CT scans showing a patient with severe IrCAAD in mid-sagittal section. b, c Showing same patient’s parasagittal and axial section, respectively. The arrow showing the abnormal areas of C1–C2 contact posterior to true facets joints. These ‘joint-like’ areas
are being labelled as ‘‘Pseudofacets’’. d–f Showing different patients with similar ‘‘pseudofacets’’ (arrows) with acute C2 faceto-isthmic angles. The arrowheads represent the true facets
section showing maximum pseudofacet. The exact dimensions of pseudofacets were difficult to measure as many of them were reconstructed images from thick slices of base CT. CT angio was obtained in the cases operated in the last 1.5 years to look for anomalous vertebral artery (VA). Operative videos and notes were reviewed at to specially study these pseudofacets. In the first 1.5 years patients with IrCAAD had undergone transoral decompression and posterior fusion. However, in the last 1.5 years we have switched over to direct posterior open reduction and fixation for these patients. Fusion was looked for in these patients at follow-up. Fusion was defined as the single cortical margin seen between C1 and C2.
within C1 (basilar invagination). Two of them also had multiple level fusions of subaxial spine. Such bony abnormalities were absent in the two patients having of RCAAD with pseudofacets (Table 1). The mean C2 faceto-isthmic angle, inferior sagittal C1 facetal angle in all patients of CAAD with pseudofacets, CAAD without pseudofacets and normal subjects are compared in Table 2. The mean C2 faceto-isthmic angle in patients of IrCAAD with pseudofacets was acute as compared to patients of IrCAAD without pseudofacets and normal subjects. Additionally, more acute the sagittal inferior C1 facetal angle, more acute was the C2 faceto-isthmic angle (Table 1).The orientation of pseudofacets in the sagittal plane was cranio-caudal anteroposteriorly, as opposed to the caudo-cranial antero-posterior sagittal inferior C1 facet or superior border of C2 facet. This orientation of the pseudofacet appeared to counter the mobility of C1–2 true facet at least in the sagittal plane. The faceto-isthmic angles in the two patients having RCAAD with pseudofacets were relatively acute as compared to those without it and normal subjects. In one of these two patients, the pseudofacets were compressing the cord. The pseudofacets appeared to be larger in these two patients with RCAAD as compared to the pseudofacets in patients with IrCAAD.
Results Out of 35 patients, pseudo facets were observed in eight patients with CAAD (Table 1). The age varied from 3 to 25 years. Pseudofacets were seen in six patients with IrCAAD (26 %) and in two patients of RCAAD (16 %). The approximate dimensions of these pseudofacets are mentioned in Table 1. All the six patients of IrCAAD with pseudofacets had occipitalised atlas, C2-3 fusion and telescoping of C2
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M/15 F/3
M/12
F/11
M/25
M/18
M/24
M/21
1 2
3
4
5
6
7
8
RCAAD (no central dislocation)
RCAAD (no central dislocation)
IrCAAD (central?ant-post)
IrCAAD (central?ant-post) with situs inversus and dextrocardia
IrCAAD (central?ant-post)
IrCAAD (central?ant-post)
IrCAAD (central?ant-post) IrCAAD (central?ant-post)
Diagnosis
–
?
?
?
?
?
? ?
Presence of occipitalised atlas and C2-3 fusion
PF with C1 lateral mass and C2 pars interarticularis screw
PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and PF with C1 lateral mass and C2 pars interarticularis screws
Direct OR and p with SLW
TOD?OC2 fusion (SLW)
TOD?OC2 fusion (SLW) TOD?OC2 fusion (SLW)
Surgery
162
152
120
146
135
145
135 110
C1 sag inferior facetal angle
86
70
62
44
61
100
72 45
C2 facetoisthmic angle
16
13
8
13
10
13
8 9
12
16
6
12
8
3
9 6
11
10
10
10
12
11
11 8
9
14
8
8
11
2
10 6
Left
Right
Right
Left
Width of pseudofacet (mm)
Length of pseudofacet (mm)
10 months
12 months
10 months
1 year
1 year
1.5 years
3 years 2 years
Follow-up
Y
Y
Y
Y
Y
Y
Y Y
C1–C2 bony fusion
IrCAAD irreducible congenital atlanto-axial dislocation, RCAAD reducible congenital atlanto-axial dislocation, central dislocation basilar invagination, TOD transoral decompression, OR open reduction, PF posterior fusion, SLW sublaminar wire
The C1 inf sagittal facetal angles and C2 faceto-isthmic angles are the average of right and left
Age/ sex
No
Table 1 Details of patients of CAAD with C1–2 ‘pseudofacets’
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Table 2 Comparison of the sagittal inferior C1 facetal angle and C2 faceto-isthmic angles in normal subjects and patients with CAAD (with and without pseudofacets) Normal [(10) Sagittal inferior C1 facetal angle
172.6 ± 4.5 (160–176)
C2 faceto-isthmic angle
135.2 ± 5 (130–145)
RCAAD without pseudofacets (10) 165.2 ± 7 (160–170) 105.8 ± 10.9 (90–120)
RCAAD with pseudofacets (2)
IrCAAD without pseudofacets (17)
IrCAAD with pseudofacets (6)
157 (152–162)
140.3 ± 18.4 (122–175)
134.4 ± 14.6 (110–150)
104.4 ± 14.2 (75–125)
64.8 ± 18.9 (40–100)
78 (70–88)
Number of patients in each subset is mentioned in parenthesis
Fig. 3 A patient of IrCAAD with pseudofacets. The upper row shows reconstructed 3DCT showing pseudofacets (marked by asterisk) and true facets (marked by arrows). The middle row shows a 2DCT image showing true (asterisk) and pseudofacets (arrow) in the same patient. The lower row shows intraoperative image showing right and left
pseudofacets (marked by asterisk) and the distracted true facets (marked arrow). The pseudofacet on left side was bulbous and smooth. This was drilled and a part was sent for histopathology that showed osseous tissue flanked by hyaline cartilage but lacked synovial membrane
There was no statistical difference in atlanto-dental interval of patients of CAAD with pseudofacets and those without it. Figure 2 depicts the psuedofacets in various patients. There was anomalous VA’s in one of the patients with IrCAAD and pseudofacets. Radiologically and intraoperatively the pseudofacets were bilateral, though asymmetrical (right and left) in all patients. The surfaces in five cases (2 with RCAAD and 3
with IrCAAD) appeared smooth with presence of cartilage (Figs. 3, 4). The presence of hyaline cartilage was proved by histopathology in two of these cases in whom the tissue was sent for histopathology. However, these lacked synovial membrane or capsule. The remaining three cases had rough surfaces and were almost in apposition. In these patients they appeared to be apposing flattened bony outgrowths. In one patient it appeared fused on the posterior
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Fig. 4 a Intraoperative images showing C1 and C2 ‘‘pseudofacets’’ (PF) with C2 spinous process (SP) in patient with RCAAD. Black arrowheads showing the C2 lamina hidden behind the gauze. b Showing C2 nerve root (C2 N) lying anterior to ‘‘pseudofacets’’ helping to differentiate true versus false facets, intraoperatively. c Shows completely opened up pseudofacets with dissector in true facets. The glistening smooth ‘‘joint-like’’ surface of C2-PF is
appreciable. d After cutting the C2 N and drilling off C1-PF the joint cleft of C1–C2 facet joint. e, f Extension-flexion CVJ X-rays of the same patient. The solid white arrows mark the pseudofacets and black thin arrows mark the true facets. g Excised specimen of pseudofacets of another patient with RCAAD. Note the smooth glistening surface (solid white arrows), resembling articulating surfaces of true synovial joint
Fig. 5 The upper row shows CT scan of a patient with severe AAD. a Preoperative image. b, c Showing post-operative images showing requirement of excessive drilling due to ‘‘pseudofacets’’ to open and reduce true facets at the cost of decreased bony purchase for C2 screws. Notice the drilled flat C1–C2 true facets with interposed bone
grafts. The complete reduction and stabilisation is seen in (d). The lower row shows another patient with ‘fixed AAD with pseudofacets’ (e) wherein ‘‘pseudofacets’’ itself are used for distraction and placement of bone graft (shown by arrow in f) and satisfactory reduction is achieved (g)
edge and could be distracted only after drilling. The C2 nerve roots were ventral to these pseudofacets and separated them from the true facets.
Of the six patients with IrCAAD with pseudofacets, transoral decompression with posterior fusion was performed in three patients. In these, posterior fusion was
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achieved with sublaminar double atlas cable. The pseudo facets were not drilled extensively as the true facets were not opened in these cases. The surfaces of C2 and occiput were decorticated to aid fusion. Direct posterior reduction and fusion were used in the remaining three patients with IrCAAD and pseudofacets. Of these three patients with IrCAAD, the pseudofacets were drilled extensively in two, to reach up to the true facets. The drilling was required because the pseudofacets came in the surgeon’s line of vision of true facets. This drilling reduced the purchase for C2 translaminar screws (Fig. 5 upper row). In the other patient (age 11), distracting these pseudofacets itself opened the true C1–2 facets and reduced the AAD to a large extent. Following this bone graft was placed between the both true and pseudofacets after drilling their surfaces to augment fusion with sublaminar wires (Fig. 5 lower row). In the patient with anomalous VA, the artery was safeguarded by dissecting it and retracting it during drilling of joint. In this case the artery was seen anterior to C2 nerve root but posterior to C1 facet. The 2 patients of RCAAD with pseudofacets underwent posterior fusion (lateral mass screws) in reduced position. In one of these two patients, the pseudofacets were causing cord compression and had to be excised. Histopathological evaluation of these pseudofacets was obtained in two cases (one with RCAAD and other with IrCAAD). It revealed osseous tissue flanked by regular hyaline cartilage but lacked synovial membrane or capsule [2]. Though the osseous tissue and the regular hyaline cartilage resembled true joint, the lack of synovium suggested it to be dysmorphic or partially formed joint. Post-operative bony fusion was seen in all the eight patients with pseudofacets. Whereas, post operative bony fusion was seen in 18 patients of the 27 with CAAD without pseudofacets. Nine patients, in whom bony fusion could not be appreciated, had no mobility on flexion extension (except one that was reoperated).
Discussion Accessory or additional joints at the CVJ, though rare have been described in patients with RCAAD [2, 3]. Histopathology had proved them to be different from true synovial joints (lack of synovial membrane) and hence we labeled them as ‘pseudofacets’. The presence of such pseudofacets’ has not been described in IrCAAD previously. They seem to have a role in imparting stability. Radiological, intraoperative and histopathological observations were analyzed in patients of CAAD with pseudofacets. Based on these observations, we have listed the possible reasons for their development and role. Besides this, their bearing on surgery and influence on outcome has been discussed.
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The C1–2 pseudofacets were more often observed in patients with acute bending of C2 lamina over facets (Fig. 1). This kind of angulation (acute C2 faceto-isthmic angle) is unlikely to increase with age and is more likely to be congenital. Additionally, it was seen in young patients (age ranged from 3 to 25 years). Thus, the presence of pseudofacets may be congenital. Embryologically, the genetic aberration of HOX genes, could have initiated the process of interzone formation at the future sites of pseudofacets. The abnormal movement at C1–2 joints would have lead to cavitation in this zone which is a future harbinger of a joint cavity [3]. The ill-sustained process (for the lack of adequate signal) may have ended up in an incomplete joint formation. The fate of the pseudofacets is decided by the embryonic time frame where the process ends. Early arrest would lead only a bony dysmorphic facet, whereas a late arrest would lead to dysmorphic facet flanked by articular cartilage without a synovial membrane and capsule [2]. Such osseous tissue with hyaline cartilage lacking synovial membrane was seen on histopathology in two of our patients. Theoretically, it is possible to have a well-formed ‘true facet joint’ with capsule and synovium at an abnormal location if process of genetic aberration is completed with proper inductive signals. The ‘genetic abberation’ theory, does not explain the variability of age at presentation of these patients. The increase in surface area of pseudofacets or even the formation of pseudofacets secondary to the CAAD cannot be ruled out. The orientation of facetal planes of atlas (C1) and axis (C2) appear to be an important factor in the antero-posterior and vertical dislocation in the Cranio-vertebral junction abnormality. The telescoping of C2 within C1 as well as the antero-posterior dislocation, possibly progresses with time (dynamic process) [1]. This continues till the portion of the C1 arch touches the C2 lamina just posterior to the isthmus, where the pseudofacets may already be existing. Furthermore, such contacts would occur earlier in patients with acute faceto-isthmic angles. This contact between C1 and C2 flattens and increases the surface area in an attempt to reduce the instability (Fig. 1c). These surfaces may fuse partially leading to C1–2 autofusion. This is akin to osteophytes in cervical spondylosis [4]. Occipitalised atlas along with C2-3 fusion increases the stress on the already malformed C1 and C2 facets, thereby augmenting the surface areas of pseudofacets to reduce the instability. Bony outgrowths are often seen at craniovertebral junction. However, these are never covered with cartilage. This increase in the surface area of pseudofacets itself may cause cord compression as was seen in one of our patients [2]. The above theory fails to explain the presence of pseudofacets in RCAAD (normal faceto-isthmic angles), though it was seen only in 2 out of 12 patients with
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RCAAD. Additionally, both patients with RCAAD had pseudofacets that simulated true facets and had larger sizes than their counterparts in IrCAAD. So it is likely to be a result of genetic aberration that is accentuated by the existing instability. Apart from this, we observed bending of C1 arch over the facets or incurving of the occiput. Unfortunately we could not define fixed points to measure them in normal subjects making it difficult to compare objectively. Whatever be the etiology, the number of contact points between C1 and C2 increases to four (two at the true facets and two at pseudofacets) thereby adding to the stability. Apart from this, the orientation and direction of pseudofacet is such that at least the antero-posterior vector at the inclined true facets is countered (Fig. 1b, c). The vertical vector remains, continuing the central dislocation. In short, ‘pseudofacets’ appear to impart stability and may be natures attempt to counter abnormal C1–2 mobility. Pseudofacets have a bearing on the management of atlanto-axial dislocations. These pseudofacets, can be distracted helping in intraoperative reduction of CAAD. These ‘pseudofacets’, by the sheer virtue of their large surface areas may actually help occipito-C2 or C1–2 bony fusion, provided they are decorticated. Even with sublaminar wires good outcome can be achieved. This makes the presence of ‘pseudofacets’ a boon. The management of IrCAAD has changed significantly in last decade. The pendulum has swung from transoral decompression to direct posterior reduction and fusion [5– 7]. Similarly, various treatment strategies for severe C1–2 subluxation have been discussed by Bach et al. [8]. Further discussion by Goel, suggests tackling the C1–2 joints even in such severe subluxations without removal of posterior arch [8, 9]. The direct posterior approach requires opening up, drilling of the true facets and placement of spacers. The anomalous VA needs to be safeguarded while the joints are being comprehensively drilled [10]. The pseudofacets come in the surgeon’s line of vision of true facets, making drilling of C1–2 true facets and placement of spacers and screws difficult. This makes their presence a ‘bane’. To reach the C1–2 facets, these pseudo-joints need to be drilled as simple distraction opens them up only transiently. The C2 nerve root is consistently found between pseudo and true facets. The inferior pseudofacet is formed by the portion of C2 lamina just posterior to the isthmus. The placement of C2 screws becomes difficult if a portion of this C2 pseudofacet is drilled as it reduces the available bony purchase. So it is preferable to drill the C1 pseudofacet. Apart from this, drilling of these pseudofacets may disrupt the nature’s compensation for instability. So this step, in a way may actually be counter-productive. The final C1–2 fusion needs to be strong.
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The postoperative bony fusion was seen only in 66 % of the patients of CAAD without pseudofacets. Whereas, postoperative bony fusion was seen in all patients of CAAD with pseudofacets. The reasons for this could be multiple. The technique of fusion has shifted to extensive decortication of facets from simple sublaminar wiring. The drilling is even more extensive in patients with pseudofacets. Secondly, the surface area of contact is more in patients with pseudofacets. Small number of patients is the limitation of this retrospective study. Additionally, the theory for formation of pseudofacets is based on a conjecture that needs to be proved by further studies. Nevertheless, it attempts to bring nature’s compensatory mechanism in CAAD to notice and that these ‘pseudofacets’ should be kept in mind while operating as they influence the surgical management and outcome. Conflict of interest
None.
Ethical standards Retrospective study of radiology and operative details of routinely operated cases of atlantoaxial dislocation.
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