The Knee 21 (2014) 1244–1249
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The Knee
Radiographic outcome of limb-based versus knee-based patient specific guides in total knee arthroplasty Catherine Crane a, Kanniraj Marimuthu b, Darren B. Chen a,b, Ian A. Harris c, Emma Wheatley d, Samuel J. MacDessi a,b,⁎ a
Department of Orthopaedic Surgery, The Canterbury Hospital, Canterbury, NSW, Australia Sydney Knee Specialists, St George Private Hospital, Sydney, NSW, Australia Ingham Institute for Applied Medical Research, South Western Sydney Clinical School, UNSW, Australia d Bryant Radiology, St George Private Hospital, Sydney, NSW, Australia b c
a r t i c l e
i n f o
Article history: Received 13 March 2014 Received in revised form 12 June 2014 Accepted 13 August 2014
Keywords: Patient specific guides Radiographic outcomes Knee arthroplasty Alignment CT Perth protocol
a b s t r a c t Background: Patient specific guides (PSG's) were developed to improve overall component alignment in total knee arthroplasty (TKA). The aim of this study was to undertake a comparative radiographic study of two commonly used PSG and determine whether the radiographic technique used to construct the PSG had a significant effect on overall alignment. Methods: This prospective cohort study examined the accuracy of limb-based (n = 112) versus knee-based (n = 105) MR PSG in restoring the mechanical axis in three planes according to post-operative Perth CT scan protocol. Results: Limb-based MR and knee-based MR PSG systems both restored overall hip–knee–ankle angle (HKAA), femoral coronal alignment, tibial coronal alignment, femoral sagittal alignment, tibial sagittal alignment and femoral rotation alignment to within 3° of a neutral mechanical axis with similar precision (91.1% vs. 86.7% p = 0.30, 97.3% vs. 96.2% p = 0.63, 97.3% vs. 97.1% p = 0.94, 94.6% vs. 89.4% p = 0.16, 90.2% vs. 81.0% p = 0.05, 91.1% vs. 86.7% p = 0.30, respectively). However, when the secondary outcome measure of alignment within 2° was assessed, limb-based MR PSG restored HKAA, femoral coronal and tibial sagittal alignment with greater precision than knee-based MR PSG (73.2% vs. 64.8% p = 0.016, 93.8% vs. 80.8% p = 0.004 and 82.1% vs. 62.9% p = 0.001, respectively). Conclusions: The findings of this study recommend the use of limb-based MR PSG for improved precision in the restoration of neutral mechanical alignment over knee-based MR PSG in TKA. Level of Evidence: Therapeutic level III © 2014 Elsevier B.V. All rights reserved.
1. Introduction A cornerstone to the success of total knee arthroplasty (TKA) is founded in component alignment. It is well documented that coronal malalignment, particularly varus malalignment is associated with increased wear, higher strain, possible premature failure of the construct and in some cases poorer outcomes [2–5]. Conventional instrumentation (CON), using a combination of intramedullary and extra-medullary jigs remains the most common technique to restore a neutral lower limb mechanical axis. Computerassisted navigation surgery (CAS) has been shown to decrease the number of outliers when compared to CON [6]. However, its popularity has not increased over recent years, potentially due to longer operative
⁎ Corresponding author at: Sydney Knee Specialists, Suite 8, 19 Kensington Street, Kogarah, NSW 2217, Australia. E-mail address: [email protected] (S.J. MacDessi).
http://dx.doi.org/10.1016/j.knee.2014.08.013 0968-0160/© 2014 Elsevier B.V. All rights reserved.
times, increased staff and equipment required in the operating room and problems associated with pin trackers [7]. Patient specific guides (PSG) are manufactured pre-operatively and are available from the majority of orthopedic implant manufacturers. These guides conform to the patient's anatomy during surgery, allowing femoral and tibial resections to be performed based on preprogrammed variables. They have been theorized to improve alignment when compared to CON, as well as reduce operative time and avoid the perceived complexity and initial set-up expenses that are often associated with CAS [8,9]. Despite these instruments being available for use for several years, there remains a paucity of data on their accuracy, especially when compared to the literature available on CON and CAS. PSG systems differ with regard to the pre-operative imaging used to define the joint topography, anatomic and mechanical axes and jig function (pinning or cutting guides). Systems based on one imaging modality (CT or MR) use a scanogram or spot scans of the hip and ankle to define the mechanical axis in addition to detailed imaging of the knee for joint topography. Other PSG systems use a combination of CT or
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MR to map the joint topography, while using plain radiography for mechanical axis restoration. At this stage, there is limited evidence to suggest that component alignment is superior with guides manufactured from a particular imaging modality. The aim of this study is to undertake a comparative radiographic study of two commonly used PSGs that utilize different radiographic techniques. The hypothesis of our study is that a whole limb-based MR PSG (LPSG) would offer better alignment in all three planes when compared to a knee-based MR PSG (KPSG) that utilizes plain radiographs for coronal reconstruction only. 2. Materials and methods A consecutive cohort of 217 patients who had undergone a TKA using PSG by two consultant surgeons (SJM, DBC) from May 2010 until March 2013 at one institution were prospectively studied. From May 2010 all patients who were consented to undergo a TKA by the consultant surgeons were eligible for inclusion in the study. Inclusion required sufficient preoperative time to manufacture PSGs and informed consent for participation in the study. Exclusion criteria included contraindications to MR examination including the presence of cardiac and cerebral implants, or metal implants close to the knee joint and other patient specific factors that inhibited MR examination. Patients who did not meet the inclusion criteria or those who met exclusion criteria underwent CON TKA. Hospital ethics board approval was gained. A total of 217 knees were replaced using MR based PSG during the study period and they comprised the two study groups. At the time of consent, patients were sequentially assigned to the study groups, firstly to KPSG then to LPSG. A formal sample size calculation was not performed for this study. Group 1 (LPSG group) included 112 consecutive patients who had undergone TKA using limb-based MR guides (Patient Specific Instrumentation (PSI), Zimmer, Warsaw, IN). The PSI system utilizes MR only, both for joint surface mapping as well as mechanical axis restoration in the coronal, sagittal, and axial planes. Group 2 (KPSG group) comprised of 105 consecutive patients who had undergone TKA using knee-based MR guides (Visionaire, Smith and Nephew, Memphis, TN). The Visionaire system utilizes a combination of MR to map the knee joint topography with long leg (hip to ankle) plain radiography to assess the femoral and tibial mechanical axis in the coronal plane. The mean age in Group 1 was 68 years (range 66 to 69 years). The mean age in Group 2 was 69 years (range 67 to 71 years). The majority of PSG TKAs in each group were performed by one surgeon SJM; 89 of 112 in Group 1 and 74 of 105 in Group 2. For both systems, a perpendicular resection in the coronal plane to the mechanical axis (MA) of the distal femur and proximal tibia was planned. In the sagittal plane, neutral femoral flexion angle was planned for the distal femur and posterior slopes of 7° and 3° were planned for the LPSG and KPSG systems respectively according to the manufacturer's recommendations. Femoral component rotation was set parallel to the surgical transepicondylar axis in both groups. In both groups, tibial rotational positioning was achieved by manually aligning the component parallel to the axis from the PCL foot print to the junction of the medial and middle one-third of the patellar ligament. The PSGs were not used in either group for tibial rotational positioning and as such were not measured as part of the analysis of the accuracy of these guides.
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sedation. A tourniquet was used for initial soft tissue dissection and then deflated for the remainder of the case. A medial parapatellar approach was undertaken for surgical exposure. The distal femoral followed by proximal tibial resections were performed using the PSGs and then the extension gap was assessed to ensure adequate bone resection and coronal alignment. The femoral preparation was then completed using the pre-drilled distal holes from the PSGs. All patients received posterior stabilized fully cemented implants using either Zimmer NexGen LPS Flex (LPSG) or Smith and Nephew Legion High Flex systems (KPSG). The patella was resurfaced in all cases. 2.2. Radiographic assessment All patients underwent a post-operative CT scan in accordance with the Perth protocol [1] using a low dose radiation of 2–3 mSv. A 2 mm slice helical scan from acetabulum to ankle joint was reconstructed to measure component alignment. All scans were analyzed by an experienced CT radiographer (EW) and a subset of 20 random scans was reviewed (KM) to test interobserver reliability. 2.3. Outcome measures The hip–knee–ankle angle (HKAA) was measured in the coronal plane as the angle between the femoral coronal mechanical axis (FMA) and the tibial coronal mechanical axis (TMA). A line was drawn from the center of rotation of the femoral head to the center of the knee on the distal femur and another line was drawn from the center of the proximal tibia to the midpoint of the talar dome of the distal tibia and the HKAA is the angle between these two lines. A valgus alignment was given a positive (+) value; a varus alignment was given a negative (−) value. HKAA was considered satisfactory if it deviated 3° or less from neutral alignment (Image A).
2.1. Surgical technique All operations were performed by two fellowship-trained knee arthroplasty surgeons (SJM, DBC) who were experienced with PSG TKA techniques. Both surgeons utilized equivalent surgical techniques. All surgeries were performed under spinal anesthetic combined with
Image A. Hip–knee–ankle angle (HKAA)
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Image C. Tibial coronal angle (TCA). Image B. Femoral coronal angle (FCA).
Femoral coronal angle (FCA) was calculated in the coronal plane as the angle between the FMA and a line through the inferior aspect of the femoral component of the knee replacement, measured on the medial aspect (Image B). Tibial coronal angle (TCA) was calculated in the coronal plane as the angle between the tibial coronal mechanical axis and a line level through the proximal aspect of the tibial component of the knee replacement, measured on the medial aspect (Image C). Femoral sagittal angle (FSA) was measured as the angle between the femoral sagittal mechanical axis and a line level with the posterior flange of the femoral component of the knee replacement. A flexed component was designated a positive value (+) and an extended component was given a negative value (−) (Image D). Tibial sagittal angle (TSA) was measured in the sagittal plane as the angle between the tibial sagittal mechanical axis and a line level with the proximal aspect of the tibial component. The target TSA was 7° in the LPSG group and 3° in the KPSG group based on manufacturer's specifications (Image E). Femoral rotation alignment (FRA) was calculated in axial sections as the angle between a line through the posterior flange of the component and the surgical transepicondylar axis. FRA was considered neutral when parallel to the surgical transepicondylar axis. A positive value (+) was assigned to indicate an externally rotated component and a negative value (−) was assigned to an internally rotated component (Image F). Tibial rotation was assessed during the CT analysis however evaluation of these values was beyond the scope of this paper. 2.4. Statistical analysis Means, standard deviations and ranges were recorded for each parameter analyzed. The proportion of cases +/−2° and +/−3° of neutral alignment for each variable was calculated, where +/− was
Image D. Femoral sagittal angle (FSA).
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Table A Patient demographics.
Age in years BMI Pre-op HKAA
LPSG (n = 112)
KPSG (n = 105)
p value
67.5 30.2 −3.2°
69.0 31.0 −2.8°
0.25 0.16 0.09
3. Results Table A summarizes the demographic profiles of the two groups. There were no significant differences between the groups with regard to age, BMI and pre-operative HKAA. 3.1. Inter-observer variability The mean difference between observers was less than 1.6° for each individual angle and the mean pooled absolute difference in angle measurements was 1.1° (SD 1.1). There was good inter-observer correlation with less than 2° difference in over 91% of angles measured; this is consistent with that described in literature [1,10,11]. A recent study published by Hauschild et al. examined intra-observer agreement values of three raters on 196 TKA post-op CT scans. Consistent with our results, the study concluded that apart from femoral posterior condylar angle (fPCA) the intra- and interobserver agreement was strong for all angles assessed [(ICC 0.2–0.6) and (ICC 0.7–1.0) respectively.] They further concluded that despite the weaker ICC values the mean absolute measurement differences for fPCA were clinically acceptable (1.2–2.6°) [12]. 3.2. Operating surgeon The number of TKAs performed according to surgeon (SJM vs. DBC) was similar between the groups; it did not reach statistical significance (p = 0.13). 3.3. Hip–knee–ankle angle
Image E. Tibial sagittal angle (TSA).
classified as less than or equal to x°. Paired student t-test and ANOVA were used to compare differences in means between continuous variables. Pearson's chi squared test was used for differences in proportions between groups for dichotomous variables. Statistical significance was defined as a p value of b0.05.
HKAA was +/−3° of a neutral mechanical axis in 91.1% of patients in the LPSG group and 86.7% in the KPSG group (p = 0.30, Table B). HKAA was +/−2° of a neutral mechanical axis in 79.5% of patients in the LPSG group and 64.8% in the KPSG group (p = 0.0016). 3.4. Femoral coronal angle FCA was +/−3° of a neutral mechanical axis in 97.3% of patients in the LPSG group and 96.2% in the KPSG group (p = 0.63). FCA was +/−2° of a neutral mechanical axis in 93.8% of patients in the LPSG group and 80.8% of patients in the KPSG group (p = 0.004). 3.5. Tibial coronal angle TCA was +/−3° of a neutral mechanical axis in 97.3% of patients in the LPSG group and 97.1% in the KPSG group (p = 0.94). TCA was +/−2° of neutral mechanical axis in 90.2% of patients in the LPSG group and 90.5% of patients in the KPSG group (p = 0.94). 3.6. Femoral sagittal angle FSA was +/−3° of a neutral mechanical axis in 94.6% of patients in the LPSG group and 89.4% in the KPSG group (p = 0.16). FSA was +/−2° of a neutral mechanical axis in 87.5% of patients in the LPSG group and 81.7% of patients in the KPSG group (p = 0.24, Table C). 3.7. Tibial sagittal angle TSA was +/−3° of a neutral mechanical axis in 90.2% of patients in the LPSG group and 81.0% in the KPSG group (p = 0.05). TSA was +/−2° of a neutral mechanical axis in 82.1% of patients in the LPSG group and 62.9% of patients in the KPSG group (p = 0.001). 3.8. Femoral rotation angle FRA was +/−3° of a neutral mechanical axis in 91.1% of patients in the LPSG group and 86.7% in the KPSG group (p = 0.30). FRA was +/−2° of a neutral mechanical axis in 79.5% of patients in the LPSG group and 74.3% of patients in the KPSG group (p = 0.37).
4. Discussion
Image F. Femoral rotation angle (FRA).
Patient specific guides are a new technology aimed at improving the alignment of components and reducing the number of alignment
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Table B Coronal alignment of components. Parameter
Mean SD Range CI 95% p value
Hip–knee–ankle angle (HKAA)
Femoral coronal angle
LPSG
KPSG
LPSG
KPSG
LPSG
KPSG
−0.2 2.2 −5 to 7 −0.6 to 0.2
0.0 2.4 −5 to 5 −0.4 to 0.5
89.6 1.4 85 to 93 89.4 to 89.9
89.6 1.8 86 to 93 89.2 to 89.9
90.0 1.6 85 to 94 89.7 to 90.3
90.4 1.5 87 to 95 90.1 to 90.7
0.54
Tibial coronal angle
0.73
0.09
+/−3° % +/−3° p value
91.1
86.7
97.3
0.30
96.2
97.3
0.63
97.1 0.94
+/−2° % +/−2° p value
73.2
64.8
93.8
0.016
outliers in patients undergoing total knee arthroplasty. PSG as a generic entity refers to a common philosophy: the use of 3-D models of the patient's knee based on pre-operative imaging to plan resections and thereby the alignment of components [13]. Although there is debate on whether a neutral mechanical axis should be a target for all individuals, restoration of a neutral mechanical alignment is still recognized as the guiding principle of knee arthroplasty surgery. There are biomechanical and clinical studies that support this tenant [2–5,19] to achieve superior outcome for patients and implants. Implant malalignment which deviates greater than +/−3° from a neutral mechanical axis has been associated with increased wear, higher implant failure and revision rates, lower knee scores and reduced patient satisfaction when compared to those +/−3° from neutral mechanical alignment [12,14,19]. The imaging modality and the method used to define the anatomic and mechanical axis vary amongst PSG systems. CT and MR based PSG systems have not been compared to determine if one modality is superior to the other. A small sample cadaveric study on femoral rotation found that CT based PSGs resulted in high deviation from the planned alignment when the cartilage was badly damaged [9]. Another study based on ovine knees found that CT based 3-D templates overestimated the size of the bone while MRI based templates were undersized. They concluded that CT based templates had more accurate bony contours than MRI based templates [15]. The preponderance of published evidence on TKA suggests that a varus/valgus deformity greater than 3° from the mechanical axis can alter the stresses encountered at the load bearing surface and compromise implant longevity [16]. Therefore, alignment +/− 3° from the Table C Sagittal and rotational alignment of components. Parameter Femoral sagittal angle LPSG Mean SD Range CI 95% p value
KPSG
Tibial sagittal angle LPSG
KPSG
Femoral rotation alignment LPSG
KPSG
0.9 1.3 1.4 1.9 −3 to 5 −4 to 9 0.6 to 1.1 0.9 to 1.6 0.09
7.0 4.1 2.0 2.5 2 to 11 −3 to 10 6.7 to 7.4 3.6 to 4.5 NC
−0.2 −0.3 2.0 2.2 −4 to 5 −5 to 6 −0.6 to 0.2 −0.7 to 0.1 0.70
94.6
89.4 0.16
90.2
81.0 0.05
91.1
86.5 0.30
87.5
81.7 0.24
82.1
62.9 0.001
79.5
74.0 0.37
+/−3° % +/−3° p value +/−2° % +/−2° p value
80.8 0.004
90.2
90.5 0.94
mechanical axis was utilized as a cut-off value for outliers and a primary outcome measure for this study. In addition, alignment +/−2° from the mechanical axis was assessed as a secondary outcome measure of the groups. Most systems use MR or CT spot scans of the hip and ankle or scanogram to define the mechanical coronal and sagittal axis. The KPSG system uses long leg AP radiographs in addition to MRI of the knee for the coronal axis. The sagittal alignment of the component is based solely on the MRI of the knee. Conteduca et al. using navigation evaluated the accuracy of tibial KPSG by comparing it to extramedullary guides and found it to be less accurate than extra-medullary guides for tibial slope [17]. The authors attributed the inaccuracy in the sagittal tibial alignment to the lack of images to define the sagittal axis. Lustig et al. evaluated the accuracy of KPSG system using intraoperative navigation and found greater variation between the planned femoral sagittal alignment and intra-operative alignment with KPSG. The authors postulated that use of knee based imaging with a 22 cm field of view (11 cm proximal and 11 cm distal to the joint line) is a likely source of error [18]. However, Vundelinckx et al. found that the mean tibial slope was closer to the target (3° posterior slope) with the KPSG system compared to the conventional guides [19]. Five different randomized trials on limb-based PSGs (three MRI based [20–22] and two CT based [23,24]) found no difference between CON TKA and PSG TKA in coronal femoral and tibial outliers. In terms of sagittal femoral and tibial outliers, all RCTs except Boonen et al. and Hamilton et al. reported no difference between the two groups. Boonen et al. and Hamilton et al. reported better accuracy with conventional guides for sagittal femoral alignment and sagittal tibial alignment, respectively. PSG groups had better alignment and less outlier for overall coronal alignment according to Ng et al., while other studies did not find any difference between the two groups. The findings of studies on CT based and MR based PSG systems were not widely different. To date, there are only two published studies that compare individual PSG systems. Victor et al. compared three LPSG and one KPSG system to CON TKA [25]. The proportion of outliers for overall coronal limb alignment, femoral component coronal, sagittal and axial alignment was not statistically different between the PSG and CON groups. The PSG group had more outliers for tibial coronal alignment and sagittal alignment than the CON group. On comparing the four PSG systems, the only difference found was that the KPSG system had a greater number of coronal limb alignment outliers and fewer sagittal femoral outliers. There was no difference between the four groups in other alignment parameters. A recent study by Ensini et al. compared the intra and post-operative alignment of two different PSG systems; KPSG and a CT based LPSG (CT-LPSG) [26]. Using intra-operative navigation, they found that the KPSG resulted in fewer femoral alignment discrepancies than CT-LPSG in the sagittal plane after application of PSG
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jigs and before the bone cuts were made. The study found no difference between the groups in terms of final prosthetic alignment in the coronal and sagittal planes as evaluated by post-operative long leg radiographs. Similar to Victor et al., we found that the use of LPSG was associated with improved overall coronal alignment when compared to KPSG. However, LPSG associated improvements of FCA and TSA documented in our study have not previously been published. These new findings may be attributed to the study design and routine use of postoperative CT scans which allow a comprehensive three dimensional evaluation of limb alignment. This study highlights the influence that preoperative imaging modality has on post-operative alignment when certain PSGs are utilized for TKA. There are some limitations to this study. As this study did not randomize allocation to the two groups, other variables (unknown confounders) may influence the outcomes measured. No cost benefit analysis was conducted and therefore the financial implications of our recommendation cannot be assessed. In addition, we did not assess tibial rotational positioning which is known to have implications on both tibiofemoral and patellofemoral kinematics. Lastly, the results of this paper found only a statistically significant difference in some secondary outcome measures. As the current literature only supports restoration of alignment +/−3° from the mechanical axis, it is uncertain whether the results of this study will be clinically relevant. The clinical implications of these findings have not been studied and therefore the practical application of these results is unknown. 5. Conclusions The aim of this study was to undertake a comparative radiographic study of two commonly used PSGs and determine whether the radiographic technique used to construct the PSG had a significant effect on overall alignment. The hypothesis was that a LPSG would offer superior reconstruction of alignment in all the three planes when compared to a KPSG. The results of our study demonstrated that overall HKAA, femoral coronal alignment and tibial sagittal alignment were restored to +/− 2° of the neutral mechanical alignment more often in the LPSG group than in the KPSG group. We found no other statistically significant differences between the groups. Therefore, if surgical preference is for a PSG TKA, we recommend the use of LPSG over KPSG for increased precision and restoration of neutral mechanical alignment. Acknowledgments No additional acknowledgements to be declared. All published authors were involved in production of this manuscript. No external assistance was sought or gained in the creation of this manuscript. No external sources of funding were accessed in the creation of this manuscript. References [1] Chauhan SK, Clark GW, Lloyd S, Scott RG, Breidahl W, Sikorski JM. Computer-assisted total knee replacement. A controlled cadaver study using a multi-parameter quantitative CT assessment of alignment (the Perth CT Protocol). J Bone Joint Surg Br 2004; 86:818–23. [2] Lombardi Jr AV, Berend KR, Ng VY. Neutral mechanical alignment: a requirement for successful TKA: affirms. Orthopedics 2011;34(9):e504–6. http://dx.doi.org/10.3928/ 01477447-20110714-40 [PubMed PMID: 21902145].
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Contents lists available at ScienceDirect
The Knee
Radiographic outcome of limb-based versus knee-based patient specific guides in total knee arthroplasty Catherine Crane a, Kanniraj Marimuthu b, Darren B. Chen a,b, Ian A. Harris c, Emma Wheatley d, Samuel J. MacDessi a,b,⁎ a
Department of Orthopaedic Surgery, The Canterbury Hospital, Canterbury, NSW, Australia Sydney Knee Specialists, St George Private Hospital, Sydney, NSW, Australia Ingham Institute for Applied Medical Research, South Western Sydney Clinical School, UNSW, Australia d Bryant Radiology, St George Private Hospital, Sydney, NSW, Australia b c
a r t i c l e
i n f o
Article history: Received 13 March 2014 Received in revised form 12 June 2014 Accepted 13 August 2014
Keywords: Patient specific guides Radiographic outcomes Knee arthroplasty Alignment CT Perth protocol
a b s t r a c t Background: Patient specific guides (PSG's) were developed to improve overall component alignment in total knee arthroplasty (TKA). The aim of this study was to undertake a comparative radiographic study of two commonly used PSG and determine whether the radiographic technique used to construct the PSG had a significant effect on overall alignment. Methods: This prospective cohort study examined the accuracy of limb-based (n = 112) versus knee-based (n = 105) MR PSG in restoring the mechanical axis in three planes according to post-operative Perth CT scan protocol. Results: Limb-based MR and knee-based MR PSG systems both restored overall hip–knee–ankle angle (HKAA), femoral coronal alignment, tibial coronal alignment, femoral sagittal alignment, tibial sagittal alignment and femoral rotation alignment to within 3° of a neutral mechanical axis with similar precision (91.1% vs. 86.7% p = 0.30, 97.3% vs. 96.2% p = 0.63, 97.3% vs. 97.1% p = 0.94, 94.6% vs. 89.4% p = 0.16, 90.2% vs. 81.0% p = 0.05, 91.1% vs. 86.7% p = 0.30, respectively). However, when the secondary outcome measure of alignment within 2° was assessed, limb-based MR PSG restored HKAA, femoral coronal and tibial sagittal alignment with greater precision than knee-based MR PSG (73.2% vs. 64.8% p = 0.016, 93.8% vs. 80.8% p = 0.004 and 82.1% vs. 62.9% p = 0.001, respectively). Conclusions: The findings of this study recommend the use of limb-based MR PSG for improved precision in the restoration of neutral mechanical alignment over knee-based MR PSG in TKA. Level of Evidence: Therapeutic level III © 2014 Elsevier B.V. All rights reserved.
1. Introduction A cornerstone to the success of total knee arthroplasty (TKA) is founded in component alignment. It is well documented that coronal malalignment, particularly varus malalignment is associated with increased wear, higher strain, possible premature failure of the construct and in some cases poorer outcomes [2–5]. Conventional instrumentation (CON), using a combination of intramedullary and extra-medullary jigs remains the most common technique to restore a neutral lower limb mechanical axis. Computerassisted navigation surgery (CAS) has been shown to decrease the number of outliers when compared to CON [6]. However, its popularity has not increased over recent years, potentially due to longer operative
⁎ Corresponding author at: Sydney Knee Specialists, Suite 8, 19 Kensington Street, Kogarah, NSW 2217, Australia. E-mail address: [email protected] (S.J. MacDessi).
http://dx.doi.org/10.1016/j.knee.2014.08.013 0968-0160/© 2014 Elsevier B.V. All rights reserved.
times, increased staff and equipment required in the operating room and problems associated with pin trackers [7]. Patient specific guides (PSG) are manufactured pre-operatively and are available from the majority of orthopedic implant manufacturers. These guides conform to the patient's anatomy during surgery, allowing femoral and tibial resections to be performed based on preprogrammed variables. They have been theorized to improve alignment when compared to CON, as well as reduce operative time and avoid the perceived complexity and initial set-up expenses that are often associated with CAS [8,9]. Despite these instruments being available for use for several years, there remains a paucity of data on their accuracy, especially when compared to the literature available on CON and CAS. PSG systems differ with regard to the pre-operative imaging used to define the joint topography, anatomic and mechanical axes and jig function (pinning or cutting guides). Systems based on one imaging modality (CT or MR) use a scanogram or spot scans of the hip and ankle to define the mechanical axis in addition to detailed imaging of the knee for joint topography. Other PSG systems use a combination of CT or
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MR to map the joint topography, while using plain radiography for mechanical axis restoration. At this stage, there is limited evidence to suggest that component alignment is superior with guides manufactured from a particular imaging modality. The aim of this study is to undertake a comparative radiographic study of two commonly used PSGs that utilize different radiographic techniques. The hypothesis of our study is that a whole limb-based MR PSG (LPSG) would offer better alignment in all three planes when compared to a knee-based MR PSG (KPSG) that utilizes plain radiographs for coronal reconstruction only. 2. Materials and methods A consecutive cohort of 217 patients who had undergone a TKA using PSG by two consultant surgeons (SJM, DBC) from May 2010 until March 2013 at one institution were prospectively studied. From May 2010 all patients who were consented to undergo a TKA by the consultant surgeons were eligible for inclusion in the study. Inclusion required sufficient preoperative time to manufacture PSGs and informed consent for participation in the study. Exclusion criteria included contraindications to MR examination including the presence of cardiac and cerebral implants, or metal implants close to the knee joint and other patient specific factors that inhibited MR examination. Patients who did not meet the inclusion criteria or those who met exclusion criteria underwent CON TKA. Hospital ethics board approval was gained. A total of 217 knees were replaced using MR based PSG during the study period and they comprised the two study groups. At the time of consent, patients were sequentially assigned to the study groups, firstly to KPSG then to LPSG. A formal sample size calculation was not performed for this study. Group 1 (LPSG group) included 112 consecutive patients who had undergone TKA using limb-based MR guides (Patient Specific Instrumentation (PSI), Zimmer, Warsaw, IN). The PSI system utilizes MR only, both for joint surface mapping as well as mechanical axis restoration in the coronal, sagittal, and axial planes. Group 2 (KPSG group) comprised of 105 consecutive patients who had undergone TKA using knee-based MR guides (Visionaire, Smith and Nephew, Memphis, TN). The Visionaire system utilizes a combination of MR to map the knee joint topography with long leg (hip to ankle) plain radiography to assess the femoral and tibial mechanical axis in the coronal plane. The mean age in Group 1 was 68 years (range 66 to 69 years). The mean age in Group 2 was 69 years (range 67 to 71 years). The majority of PSG TKAs in each group were performed by one surgeon SJM; 89 of 112 in Group 1 and 74 of 105 in Group 2. For both systems, a perpendicular resection in the coronal plane to the mechanical axis (MA) of the distal femur and proximal tibia was planned. In the sagittal plane, neutral femoral flexion angle was planned for the distal femur and posterior slopes of 7° and 3° were planned for the LPSG and KPSG systems respectively according to the manufacturer's recommendations. Femoral component rotation was set parallel to the surgical transepicondylar axis in both groups. In both groups, tibial rotational positioning was achieved by manually aligning the component parallel to the axis from the PCL foot print to the junction of the medial and middle one-third of the patellar ligament. The PSGs were not used in either group for tibial rotational positioning and as such were not measured as part of the analysis of the accuracy of these guides.
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sedation. A tourniquet was used for initial soft tissue dissection and then deflated for the remainder of the case. A medial parapatellar approach was undertaken for surgical exposure. The distal femoral followed by proximal tibial resections were performed using the PSGs and then the extension gap was assessed to ensure adequate bone resection and coronal alignment. The femoral preparation was then completed using the pre-drilled distal holes from the PSGs. All patients received posterior stabilized fully cemented implants using either Zimmer NexGen LPS Flex (LPSG) or Smith and Nephew Legion High Flex systems (KPSG). The patella was resurfaced in all cases. 2.2. Radiographic assessment All patients underwent a post-operative CT scan in accordance with the Perth protocol [1] using a low dose radiation of 2–3 mSv. A 2 mm slice helical scan from acetabulum to ankle joint was reconstructed to measure component alignment. All scans were analyzed by an experienced CT radiographer (EW) and a subset of 20 random scans was reviewed (KM) to test interobserver reliability. 2.3. Outcome measures The hip–knee–ankle angle (HKAA) was measured in the coronal plane as the angle between the femoral coronal mechanical axis (FMA) and the tibial coronal mechanical axis (TMA). A line was drawn from the center of rotation of the femoral head to the center of the knee on the distal femur and another line was drawn from the center of the proximal tibia to the midpoint of the talar dome of the distal tibia and the HKAA is the angle between these two lines. A valgus alignment was given a positive (+) value; a varus alignment was given a negative (−) value. HKAA was considered satisfactory if it deviated 3° or less from neutral alignment (Image A).
2.1. Surgical technique All operations were performed by two fellowship-trained knee arthroplasty surgeons (SJM, DBC) who were experienced with PSG TKA techniques. Both surgeons utilized equivalent surgical techniques. All surgeries were performed under spinal anesthetic combined with
Image A. Hip–knee–ankle angle (HKAA)
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Image C. Tibial coronal angle (TCA). Image B. Femoral coronal angle (FCA).
Femoral coronal angle (FCA) was calculated in the coronal plane as the angle between the FMA and a line through the inferior aspect of the femoral component of the knee replacement, measured on the medial aspect (Image B). Tibial coronal angle (TCA) was calculated in the coronal plane as the angle between the tibial coronal mechanical axis and a line level through the proximal aspect of the tibial component of the knee replacement, measured on the medial aspect (Image C). Femoral sagittal angle (FSA) was measured as the angle between the femoral sagittal mechanical axis and a line level with the posterior flange of the femoral component of the knee replacement. A flexed component was designated a positive value (+) and an extended component was given a negative value (−) (Image D). Tibial sagittal angle (TSA) was measured in the sagittal plane as the angle between the tibial sagittal mechanical axis and a line level with the proximal aspect of the tibial component. The target TSA was 7° in the LPSG group and 3° in the KPSG group based on manufacturer's specifications (Image E). Femoral rotation alignment (FRA) was calculated in axial sections as the angle between a line through the posterior flange of the component and the surgical transepicondylar axis. FRA was considered neutral when parallel to the surgical transepicondylar axis. A positive value (+) was assigned to indicate an externally rotated component and a negative value (−) was assigned to an internally rotated component (Image F). Tibial rotation was assessed during the CT analysis however evaluation of these values was beyond the scope of this paper. 2.4. Statistical analysis Means, standard deviations and ranges were recorded for each parameter analyzed. The proportion of cases +/−2° and +/−3° of neutral alignment for each variable was calculated, where +/− was
Image D. Femoral sagittal angle (FSA).
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Table A Patient demographics.
Age in years BMI Pre-op HKAA
LPSG (n = 112)
KPSG (n = 105)
p value
67.5 30.2 −3.2°
69.0 31.0 −2.8°
0.25 0.16 0.09
3. Results Table A summarizes the demographic profiles of the two groups. There were no significant differences between the groups with regard to age, BMI and pre-operative HKAA. 3.1. Inter-observer variability The mean difference between observers was less than 1.6° for each individual angle and the mean pooled absolute difference in angle measurements was 1.1° (SD 1.1). There was good inter-observer correlation with less than 2° difference in over 91% of angles measured; this is consistent with that described in literature [1,10,11]. A recent study published by Hauschild et al. examined intra-observer agreement values of three raters on 196 TKA post-op CT scans. Consistent with our results, the study concluded that apart from femoral posterior condylar angle (fPCA) the intra- and interobserver agreement was strong for all angles assessed [(ICC 0.2–0.6) and (ICC 0.7–1.0) respectively.] They further concluded that despite the weaker ICC values the mean absolute measurement differences for fPCA were clinically acceptable (1.2–2.6°) [12]. 3.2. Operating surgeon The number of TKAs performed according to surgeon (SJM vs. DBC) was similar between the groups; it did not reach statistical significance (p = 0.13). 3.3. Hip–knee–ankle angle
Image E. Tibial sagittal angle (TSA).
classified as less than or equal to x°. Paired student t-test and ANOVA were used to compare differences in means between continuous variables. Pearson's chi squared test was used for differences in proportions between groups for dichotomous variables. Statistical significance was defined as a p value of b0.05.
HKAA was +/−3° of a neutral mechanical axis in 91.1% of patients in the LPSG group and 86.7% in the KPSG group (p = 0.30, Table B). HKAA was +/−2° of a neutral mechanical axis in 79.5% of patients in the LPSG group and 64.8% in the KPSG group (p = 0.0016). 3.4. Femoral coronal angle FCA was +/−3° of a neutral mechanical axis in 97.3% of patients in the LPSG group and 96.2% in the KPSG group (p = 0.63). FCA was +/−2° of a neutral mechanical axis in 93.8% of patients in the LPSG group and 80.8% of patients in the KPSG group (p = 0.004). 3.5. Tibial coronal angle TCA was +/−3° of a neutral mechanical axis in 97.3% of patients in the LPSG group and 97.1% in the KPSG group (p = 0.94). TCA was +/−2° of neutral mechanical axis in 90.2% of patients in the LPSG group and 90.5% of patients in the KPSG group (p = 0.94). 3.6. Femoral sagittal angle FSA was +/−3° of a neutral mechanical axis in 94.6% of patients in the LPSG group and 89.4% in the KPSG group (p = 0.16). FSA was +/−2° of a neutral mechanical axis in 87.5% of patients in the LPSG group and 81.7% of patients in the KPSG group (p = 0.24, Table C). 3.7. Tibial sagittal angle TSA was +/−3° of a neutral mechanical axis in 90.2% of patients in the LPSG group and 81.0% in the KPSG group (p = 0.05). TSA was +/−2° of a neutral mechanical axis in 82.1% of patients in the LPSG group and 62.9% of patients in the KPSG group (p = 0.001). 3.8. Femoral rotation angle FRA was +/−3° of a neutral mechanical axis in 91.1% of patients in the LPSG group and 86.7% in the KPSG group (p = 0.30). FRA was +/−2° of a neutral mechanical axis in 79.5% of patients in the LPSG group and 74.3% of patients in the KPSG group (p = 0.37).
4. Discussion
Image F. Femoral rotation angle (FRA).
Patient specific guides are a new technology aimed at improving the alignment of components and reducing the number of alignment
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Table B Coronal alignment of components. Parameter
Mean SD Range CI 95% p value
Hip–knee–ankle angle (HKAA)
Femoral coronal angle
LPSG
KPSG
LPSG
KPSG
LPSG
KPSG
−0.2 2.2 −5 to 7 −0.6 to 0.2
0.0 2.4 −5 to 5 −0.4 to 0.5
89.6 1.4 85 to 93 89.4 to 89.9
89.6 1.8 86 to 93 89.2 to 89.9
90.0 1.6 85 to 94 89.7 to 90.3
90.4 1.5 87 to 95 90.1 to 90.7
0.54
Tibial coronal angle
0.73
0.09
+/−3° % +/−3° p value
91.1
86.7
97.3
0.30
96.2
97.3
0.63
97.1 0.94
+/−2° % +/−2° p value
73.2
64.8
93.8
0.016
outliers in patients undergoing total knee arthroplasty. PSG as a generic entity refers to a common philosophy: the use of 3-D models of the patient's knee based on pre-operative imaging to plan resections and thereby the alignment of components [13]. Although there is debate on whether a neutral mechanical axis should be a target for all individuals, restoration of a neutral mechanical alignment is still recognized as the guiding principle of knee arthroplasty surgery. There are biomechanical and clinical studies that support this tenant [2–5,19] to achieve superior outcome for patients and implants. Implant malalignment which deviates greater than +/−3° from a neutral mechanical axis has been associated with increased wear, higher implant failure and revision rates, lower knee scores and reduced patient satisfaction when compared to those +/−3° from neutral mechanical alignment [12,14,19]. The imaging modality and the method used to define the anatomic and mechanical axis vary amongst PSG systems. CT and MR based PSG systems have not been compared to determine if one modality is superior to the other. A small sample cadaveric study on femoral rotation found that CT based PSGs resulted in high deviation from the planned alignment when the cartilage was badly damaged [9]. Another study based on ovine knees found that CT based 3-D templates overestimated the size of the bone while MRI based templates were undersized. They concluded that CT based templates had more accurate bony contours than MRI based templates [15]. The preponderance of published evidence on TKA suggests that a varus/valgus deformity greater than 3° from the mechanical axis can alter the stresses encountered at the load bearing surface and compromise implant longevity [16]. Therefore, alignment +/− 3° from the Table C Sagittal and rotational alignment of components. Parameter Femoral sagittal angle LPSG Mean SD Range CI 95% p value
KPSG
Tibial sagittal angle LPSG
KPSG
Femoral rotation alignment LPSG
KPSG
0.9 1.3 1.4 1.9 −3 to 5 −4 to 9 0.6 to 1.1 0.9 to 1.6 0.09
7.0 4.1 2.0 2.5 2 to 11 −3 to 10 6.7 to 7.4 3.6 to 4.5 NC
−0.2 −0.3 2.0 2.2 −4 to 5 −5 to 6 −0.6 to 0.2 −0.7 to 0.1 0.70
94.6
89.4 0.16
90.2
81.0 0.05
91.1
86.5 0.30
87.5
81.7 0.24
82.1
62.9 0.001
79.5
74.0 0.37
+/−3° % +/−3° p value +/−2° % +/−2° p value
80.8 0.004
90.2
90.5 0.94
mechanical axis was utilized as a cut-off value for outliers and a primary outcome measure for this study. In addition, alignment +/−2° from the mechanical axis was assessed as a secondary outcome measure of the groups. Most systems use MR or CT spot scans of the hip and ankle or scanogram to define the mechanical coronal and sagittal axis. The KPSG system uses long leg AP radiographs in addition to MRI of the knee for the coronal axis. The sagittal alignment of the component is based solely on the MRI of the knee. Conteduca et al. using navigation evaluated the accuracy of tibial KPSG by comparing it to extramedullary guides and found it to be less accurate than extra-medullary guides for tibial slope [17]. The authors attributed the inaccuracy in the sagittal tibial alignment to the lack of images to define the sagittal axis. Lustig et al. evaluated the accuracy of KPSG system using intraoperative navigation and found greater variation between the planned femoral sagittal alignment and intra-operative alignment with KPSG. The authors postulated that use of knee based imaging with a 22 cm field of view (11 cm proximal and 11 cm distal to the joint line) is a likely source of error [18]. However, Vundelinckx et al. found that the mean tibial slope was closer to the target (3° posterior slope) with the KPSG system compared to the conventional guides [19]. Five different randomized trials on limb-based PSGs (three MRI based [20–22] and two CT based [23,24]) found no difference between CON TKA and PSG TKA in coronal femoral and tibial outliers. In terms of sagittal femoral and tibial outliers, all RCTs except Boonen et al. and Hamilton et al. reported no difference between the two groups. Boonen et al. and Hamilton et al. reported better accuracy with conventional guides for sagittal femoral alignment and sagittal tibial alignment, respectively. PSG groups had better alignment and less outlier for overall coronal alignment according to Ng et al., while other studies did not find any difference between the two groups. The findings of studies on CT based and MR based PSG systems were not widely different. To date, there are only two published studies that compare individual PSG systems. Victor et al. compared three LPSG and one KPSG system to CON TKA [25]. The proportion of outliers for overall coronal limb alignment, femoral component coronal, sagittal and axial alignment was not statistically different between the PSG and CON groups. The PSG group had more outliers for tibial coronal alignment and sagittal alignment than the CON group. On comparing the four PSG systems, the only difference found was that the KPSG system had a greater number of coronal limb alignment outliers and fewer sagittal femoral outliers. There was no difference between the four groups in other alignment parameters. A recent study by Ensini et al. compared the intra and post-operative alignment of two different PSG systems; KPSG and a CT based LPSG (CT-LPSG) [26]. Using intra-operative navigation, they found that the KPSG resulted in fewer femoral alignment discrepancies than CT-LPSG in the sagittal plane after application of PSG
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jigs and before the bone cuts were made. The study found no difference between the groups in terms of final prosthetic alignment in the coronal and sagittal planes as evaluated by post-operative long leg radiographs. Similar to Victor et al., we found that the use of LPSG was associated with improved overall coronal alignment when compared to KPSG. However, LPSG associated improvements of FCA and TSA documented in our study have not previously been published. These new findings may be attributed to the study design and routine use of postoperative CT scans which allow a comprehensive three dimensional evaluation of limb alignment. This study highlights the influence that preoperative imaging modality has on post-operative alignment when certain PSGs are utilized for TKA. There are some limitations to this study. As this study did not randomize allocation to the two groups, other variables (unknown confounders) may influence the outcomes measured. No cost benefit analysis was conducted and therefore the financial implications of our recommendation cannot be assessed. In addition, we did not assess tibial rotational positioning which is known to have implications on both tibiofemoral and patellofemoral kinematics. Lastly, the results of this paper found only a statistically significant difference in some secondary outcome measures. As the current literature only supports restoration of alignment +/−3° from the mechanical axis, it is uncertain whether the results of this study will be clinically relevant. The clinical implications of these findings have not been studied and therefore the practical application of these results is unknown. 5. Conclusions The aim of this study was to undertake a comparative radiographic study of two commonly used PSGs and determine whether the radiographic technique used to construct the PSG had a significant effect on overall alignment. The hypothesis was that a LPSG would offer superior reconstruction of alignment in all the three planes when compared to a KPSG. The results of our study demonstrated that overall HKAA, femoral coronal alignment and tibial sagittal alignment were restored to +/− 2° of the neutral mechanical alignment more often in the LPSG group than in the KPSG group. We found no other statistically significant differences between the groups. Therefore, if surgical preference is for a PSG TKA, we recommend the use of LPSG over KPSG for increased precision and restoration of neutral mechanical alignment. Acknowledgments No additional acknowledgements to be declared. All published authors were involved in production of this manuscript. No external assistance was sought or gained in the creation of this manuscript. No external sources of funding were accessed in the creation of this manuscript. References [1] Chauhan SK, Clark GW, Lloyd S, Scott RG, Breidahl W, Sikorski JM. Computer-assisted total knee replacement. A controlled cadaver study using a multi-parameter quantitative CT assessment of alignment (the Perth CT Protocol). J Bone Joint Surg Br 2004; 86:818–23. [2] Lombardi Jr AV, Berend KR, Ng VY. Neutral mechanical alignment: a requirement for successful TKA: affirms. Orthopedics 2011;34(9):e504–6. http://dx.doi.org/10.3928/ 01477447-20110714-40 [PubMed PMID: 21902145].
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