ORIGINAL ARTICLE
Biomechanical Comparison of Perpendicular Versus Oblique In Situ Screw Fixation of Slipped Capital Femoral Epiphysis Michael K. Merz, MD,* Farid Amirouche, PhD,* Giovanni F. Solitro, PhD,* Jeffrey A. Silverstein, MD,w Tyler Surma, BS,* and Prasad V. Gourineni, MD*z
Background: Percutaneous in situ single screw fixation is the preferred treatment for stable and unstable slipped capital femoral epiphysis (SCFE). The recommended screw placement is in the center of the epiphysis and perpendicular to the physis, which necessitates an anterior starting point for most SCFEs. A recent clinical study has shown good clinical results with a laterally based screw for SCFE, which is oblique to the physis. We sought to biomechanically compare these 2 techniques for load to failure and hypothesized that the laterally based oblique screw is equivalent or superior to an anteriorly based perpendicular screw. Methods: Twenty-two paired immature porcine femurs were used to compare the techniques. A SCFE model was created in all femurs using a previously published technique by performing a 30-degree posterior closing wedge osteotomy through the proximal physis. In the control group, a screw was placed perpendicular to the slip with an anterior starting point. In the experimental group, the screw was started as close to the midlateral cortex of the proximal femur as possible while maintaining the screw anterior to the posterior cortex of the femoral neck and ending at the apex of the epiphysis ignoring the resultant angle to the physis for the experimental group. The specimens were then potted and loaded in a physiologically relevant posteroinferior direction (30 degrees posterior from vertical) to determine load to failure (N) and stiffness (N/mm). Results: No statistical difference was found between the 2 groups in maximum load to failure or stiffness (P > 0.05). Conclusions: A laterally based screw oblique to the physis for in situ fixation in mild SCFE is not significantly different than an anteriorly based screw perpendicular to the physis in load to failure and stiffness in our study model. Clinical Relevance: In light of no difference in load to failure of these 2 constructs, surgeons may be more comfortable with the traditional lateral entry point while still aiming for screw placement in the center of head.
From the *Department of Orthopaedics, University of Illinois at Chicago, Chicago, IL; wSarasota Orthopedic Associates, Sarasota; and zAdvocate Children’s Hospital, Oak Lawn, IL. A Synthes educational grant donated all screws used in this study. The authors declare no conflicts of interest. Reprints: Michael K. Merz, MD, 835 S. Wolcott Avenue, M/C 844, Rm E270, Chicago, IL 60612. E-mail: [email protected]. Copyright r 2014 Wolters Kluwer Health, Inc. All rights reserved.
816 | www.pedorthopaedics.com
Key Words: slipped capital femoral epiphysis, pinning, biomechanical (J Pediatr Orthop 2015;35:816–820)
S
lipped capital femoral epiphysis (SCFE) is a common adolescent hip disorder with an incidence of 10.8 per 100,000 children in the United States.1 Approximately 20% of patients presenting with SCFE have bilateral involvement and another 20% to 40% are likely to progress to bilateral involvement.2,3 This accounts for a significant surgical burden of this disease worldwide. The gold standard for treatment of stable and unstable types of SCFE as defined by Loder et al4 is with percutaneous in situ screw fixation to prevent slip progression and to obtain physeal closure.5–7 Southwick angle has also been used to delineate mild (50 degrees) slips in SCFE.8 A mild slip as defined by Southwick is treated with a single in situ percutaneous screw. The current recommendation is to place a single screw into the center of the femoral head and perpendicular to the physis.9 Eccentric screw placement was shown to delay physeal closure6 and allow progression of the slip.5 A recently published case series10 described the results of 36 stable SCFE hips, which were treated with a lateral screw oblique to the physis instead of the standard anterior and perpendicular screw. No screw-related complications were noted and an adequate number of screw threads were able to fit proximal to the physis. The lateral technique is described as technically less demanding, which decreases the risk of fracture due to multiple passes with the drill11 and impingement of the screw head.12 In addition, it was theorized that the lateral starting point has stronger bone than the anteriorly based screw, which would decrease the risk of neck plow from a “windshield wiper” effect as described by Upasani et al13 and ultimately slip progression. We sought to create a biomechanical model to test the laterally based oblique screw compared with the gold standard anteriorly based perpendicular screw. We hypothesize that the lateral screw has superior load to failure than the anterior screw. If clinically equivalent or superior, the lateral technique of screw placement could J Pediatr Orthop
Volume 35, Number 8, December 2015
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
J Pediatr Orthop
Volume 35, Number 8, December 2015
offer a safer and less challenging way for orthopaedic surgeons to treat mild stable SCFE patients. Moreover, the similarity of the lateral technique to that of percutaneous pinning for femoral neck fractures in adults may make this technique more straightforward for orthopaedic surgeons who do not have a pediatric orthopaedic specialist to treat these patients in their area.
METHODS An initial pilot study was performed using 22 adult left femur Sawbones (Pacific Research Laboratories Inc., Vashon Island, WA). An Instron (Instron, Norwood, MA) loading machine was used to place an inferiorly directed force to failure on the specimens with either a point load using a sphere or a flat loading surface. Failure was defined as 10 mm of displacement of the femoral head. We repeated the experiment using immature porcine femora as described previously.13–15 Twenty-two skeletally immature (11 to 12 wk old), fresh, female porcine femora were obtained from 11 specimens. Previous work by Speer16 has shown the mechanical strength of the porcine bone is similar to an adolescent human. Soft tissues were stripped from the specimens and a posterior 30-degree angular wedge was resected from the femoral neck to create a mild SCFE. Each paired set of femora were tested against each other alternating the type of screw placed on each side. We initially used an opening wedge SCFE model in the Sawbones trial as it more closely resembles the pathology in SCFE, but as the closing wedge model is the most commonly published,13–15 we felt that using a closing wedge would have more results to compare against as an additional validation of our model and results. A guidewire was placed in a retrograde manner perpendicular to the physeal cut with an anterior starting point in the control group (Fig. 1). The accuracy of the placement was verified with fluoroscopy and measurement on anterior and frog-leg lateral views before screw insertion. A Synthes (Johnson & Johnson, West Chester, PA) 7.3 mm stainless steel cannulated screw with 32 mm partial threads was used for all specimens. The screw was advanced to the subchondral bone of the femoral head, which was verified both grossly and fluoroscopically. The laterally based oblique screws were inserted in a similar manner (Fig. 2). The starting point was on the lateral cortex proximal to the lesser trochanter aiming at the center of the head. Likewise, the screw was advanced to, but not through, the subchondral bone and verified grossly and fluoroscopically. Of note, only 1 length of screw (60 mm) was available for all specimens and therefore the head of the screw was proud on all specimens. This was not thought to affect the strength of the construct because it was the same for all specimens and compression of the physis has been disproven as an important aspect of fixation for SCFE.13 All specimens were potted in plaster of Paris distal to the lesser trochanter tilting 30 degrees posterior from the vertical axis. These were placed in an Instron loading Copyright
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Percutaneous In Situ Screw Fixation for SCFE
machine with a directly inferior direction at 5 mm/min instead of using 30 mm/min as previously described13–15 to more precisely observe the behavior for the samples. Similar to other studies, failure was defined as displacement of the femoral head of 10 mm.13–15 Maximum load to failure (N) and forces (N) were recorded every 100 ms at 2 N increment up to 8 N using the Merlin software (Instron). Stiffness (N/mm) was also determined using the computed average slope of the load displacement curve. Mean and SD were calculated for each group and compared using a 2-tailed Student t test.
RESULTS For the Sawbones trial, cantilever bending was noticed in the diaphysis during loading, particularly when the point load was applied. All Sawbones specimens were potted with approximately 5 cm of diaphysis exposed distal to the lesser trochanter. To eliminate cantilever bending in the porcine model, all specimens were potted up to the lesser trochanter. For the porcine model, the perpendicular screw construct had an average maximum failure load of 989.4 ± 339.3 N, and the oblique screw construct had an average maximum failure load of 916.2 ± 271.1 N. The maximum load to failure was not statistically significant between screw orientations (P = 0.62). The average forces at 2.5 mm of displacement were 386.8 ± 75.7 and 407.9 ± 69.3 N for the perpendicular and oblique constructs, respectively (P = 0.55). At 5 mm of displacement the average forces were 619.4 ± 194.2 and 640.3 ± 131.6 N for the perpendicular and oblique constructs, respectively (P = 0.79). The stiffness was estimated using the linear regression intercepting the origin of the obtained average curves, which had no significant difference: 107.8 N/mm (R2 = 0.94) for the perpendicular and 108.2 N/mm (R2 = 0.85) for the oblique screw construct. Without imposing origin interception the linear regression had higher coefficient of determination with values of stiffness of same order: 92.8 N/mm (R2 = 0.98) for the perpendicular and 85.9 (R2 = 0.94) for the oblique in situ fixation. Furthermore, the slope that best represents the tangent to the regression line not passing by the origin had: (load = 101.6+92.8 displacement) for perpendicular and (load = 151.9+85.9 displacement) for oblique. Graphical representation comparing displacement and load for the 2 models and their SDs is shown in Figure 3. The primary mode of failure for all specimens was due to head plow for both constructs. Head plow has been described previously as failure of fixation from movement of the femoral head inferiorly over a stable screw parallel to the physis, and is one of the most common modes of construct failure.13 No specimens failed due to fracture of the femoral head or neck.
DISCUSSION This study compared the effect of the direction of screw placement on fixation strength for in situ fixation www.pedorthopaedics.com |
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
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FIGURE 1. Anterior (A) and lateral (B) view of porcine model with screw starting anteriorly and oriented perpendicular to the physis.
of a mild SCFE using a porcine model. To the best of our knowledge, no biomechanical study has compared these techniques using physiological loads. Previous clinical
studies have established that single screw fixation is both efficacious and safer in treatment of SCFE than 2 screws.6,7
FIGURE 2. Anterior (A) and lateral (B) view of porcine model with screw starting laterally and oriented oblique to the physis.
818 | www.pedorthopaedics.com
Copyright
r
2014 Wolters Kluwer Health, Inc. All rights reserved.
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
J Pediatr Orthop
Volume 35, Number 8, December 2015
Biomechanical Comparison of Screw Fixations 1200 Oblique in situ screw fixation Perpendicular in situ screw fixation
1000 Applied Load [N]
Standard deviation for Oblique Standard deviation for Perpendicular
800 600 400 200 0 0
2
4
6
8
10
Displacement [mm]
FIGURE 3. Graph comparing fixation strength of the porcine models between perpendicular and oblique screw fixation including SD.
Multiple biomechanical studies have evaluated different techniques for in situ fixation of SCFE. They have shown increased load to failure14,17 and increased torsional strength17,18 using 2 screws compared with 1 screw; however, all studies caution against using 2 screws due to the danger of penetration into the joint. A previous study19 recommended compression across the physis with screw fixation because of a 47% increase in stiffness, but did not show differences in load to failure between compression and noncompression groups. A subsequent study13 using a porcine model under physiological loads showed a statistically significant greater load to failure and stiffness for screws placed with an even number of threads on both sides of the physis compared with more threads across the physis. Fully versus partially threaded screws have also been compared and ultimately found to have no significant difference in load to failure.15 However, in situ screw fixation does not address the residual deformity of the proximal femur. The residual deformity can cause hip impingement20,21 and arthritis in the long term, especially in the more severe slips.22 Similarly, in Gourineni’s10 study, the laterally based oblique screw was initially used because it was noticed that the residual deformity of in situ fixation of SCFE caused impingement and arthritis in more severe slips. As a result, the technique was modified to allow concurrent resection of the anterior femoral neck prominence without risking screw failure which has been described by Leunig et al.23 In this study we endeavored to determine whether there is a difference between a lateral screw oblique to the physis compared with an anterior perpendicular screw for maximum load to failure and stiffness. Our study showed no significant difference between these 2 techniques for in situ fixation of a mild SCFE in the porcine model. We used an immature porcine model previously described with a similar number of specimens for each treatment group.13–15,17,18 Our values for maximum load to failure and stiffness are comparable with those for a single-crew construct in previous studies,13–15 and therefore we feel our model was appropriate. For the porcine model, we Copyright
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Percutaneous In Situ Screw Fixation for SCFE
chose only to use a distributed load instead of the point load because the distributed load more closely represents the conformity between the acetabulum and femoral head. Walters and Simon24 described a “blind spot” radiographically on the head of the femur that can lead to unrecognized screw penetration and ultimately chondrolysis with perpendicularly oriented screws. Jofe et al25 described their experience with 17 patients who had chondrolysis after in situ fixation of a SCFE and found that 15 (88%) patients had obvious signs of intra-articular penetration with the remaining 2 patients’ screws within 2 mm of the articular surface on radiographs. These 2 studies highlight the relation between screw penetration and chondrolysis as well as the difficulty in intraoperatively determining whether or not screw penetration has occurred. We theorize that using a laterally based screw may have less intra-articular penetration due to the perpendicular orientation of the screw to an anteroposterior radiograph taken intraoperatively. This would avoid the “blind spot” and make any penetration more readily apparent on radiographs. Our study has several limitations. It is a cadaveric animal model treated in vitro attempting to reproduce an in vivo condition. Porcine femora have been shown to be suitable for SCFE models, but there are inherent differences in size and morphology compared with an adolescent human. Having matched pairs of femora from the same animal reduces the amount of heterogeneity between specimens. Similarly, all specimens were of the same sex and within 1 week of the same age at the time of sacrifice. Another limitation is that the load to failure does not represent the cyclical loading of the hip experienced in vivo. A single screw length available for all specimens also may have affected load to failure particularly in the perpendicular screws. There was a greater length of screw in the femoral bone for the lateral screw models. We initially hypothesized that the lateral screw may be superior to the anterior screw partially due to this difference in screw length. The longer screw with a longer lever arm and support in the bone may lead to superior loads to failure in the lateral group. In our study though there was no significant difference between the 2. Potentially in a larger model, the longer lever arm would have more of an effect. It was noted when the femurs were sectioned after loading that some perpendicular specimens had threads engaging the anterior cortex of the femoral neck which was not seen in any of the femurs with oblique screws. This may have created a stronger construct for the perpendicular screw due to more cortical fixation of the screw. However, we do not consider the threads engaging the anterior cortex to influence plow in the anterior screw models for this study. Had the lateral screw models failed due to neck plow instead of head plow, we may have considered this as a difference between the specimens. In addition, all specimens of both groups failed due to head plow. The anterior screw models with threads engaging the anterior femoral neck cortex did not statistically significantly affect load to failure. www.pedorthopaedics.com |
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CONCLUSIONS Laterally based oblique in situ screw fixation of stable SCFE is not significantly different in load to failure and stiffness compared with an anteriorly based perpendicular screw. In light of a recent clinical series showing no complications with this technique, it is a reasonable alternative to a perpendicularly placed screw. It may have the additional benefit of decreased screw penetration because it is outside the “blind spot” and perpendicularly oriented to an anteroposterior radiograph intraoperatively. In addition, it is a familiar technique due to its similarity to percutaneous pinning of adult femoral neck fractures for the general orthopaedic surgeon who may not have a pediatric orthopaedic specialist to treat these patients in their area. REFERENCES 1. Lehmann CL, Arons RR, Loder RT, et al. The epidemiology of slipped capital femoral epiphysis: an update. J Pediatr Orthop. 2006;26:286–290. 2. Riad J, Bajelidze G, Gabos PG. Bilateral slipped capital femoral epiphysis: predictive factors for contralateral slip. J Pediatr Orthop. 2007;27:411–414. 3. Hagglund G, Hansson LI, Ordeberg G, et al. Bilaterality in slipped upper femoral epiphysis. J Bone Joint Surg Br. 1988;70:179–181. 4. Loder RT, Richards BS, Shapiro PS, et al. Acute slipped capital femoral epiphysis: the importance of physeal stability. J Bone Joint Surg Am. 1993;75:1134–1140. 5. Aronson DD, Carlson WE. Slipped capital femoral epiphysis. A prospective study of fixation with a single screw. J Bone Joint Surg Am. 1992;74:810–819. 6. Ward WT, Stefko J, Wood KB, et al. Fixation with a single screw for slipped capital femoral epiphysis. J Bone Joint Surg Am. 1992;74: 799–809. 7. Carney BT, Birbaum P, Minter C. Slip progression after in situ single screw fixation for stable slipped capital femoral epiphysis. J Pediatr Orthop. 2003;23:584–589. 8. Southwick WO. Osteotomy through the lesser trochanter for slipped capital femoral epiphysis. J Bone Joint Surg Am. 1967;49:807–835. 9. Herring JA. Tachdjian’s Pediatric Orthopaedics. 4th ed. Philadelphia, PA: Saunders Elsevier; 2008:853–854. 10. Gourineni P. Oblique in situ screw fixation of stable slipped capital femoral epiphysis. J Pediatr Orthop. 2013;33:135–138.
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11. Canale ST, Azar F, Young J, et al. Subtrochanteric fracture after fixation of slipped capital femoral epiphysis: a complication of unused drill holes. J Pediatr Orthop. 1994;14:623–626. 12. Goodwin RC, Mahar AT, Oswald TS, et al. Screw head impingement after in situ fixation in moderate and severe slipped capital femoral epiphysis. J Pediatr Orthop. 2007;27:319–325. 13. Upasani V, Kishan S, Oka R, et al. Biomechanical analysis of single screw fixation for slipped capital femoral epiphysis: are more threads across the physis necessary for stability? J Pediatr Orthop. 2006;26:474–478. 14. Kishan S, Upsani V, Mahar A, et al. Biomechanical stability of single-screw vs. two-screw fixation of an unstable slipped capital femoral epiphysis model: effect of screw position in the femoral neck. J Pediatr Orthop. 2006;26:601–605. 15. Miyanji F, Mahar A, Oka R, et al. Biomechanical comparison of fully and partially threaded screws for fixation of slipped capital femoral epiphysis. J Pediatr Orthop. 2008;28:49–52. 16. Speer DP. The John Charnley Award Paper. Experimental epiphysiolysis: etiologic models slipped capital femoral epiphysis. Hip. 1982:68–88. 17. Segal LS, Jacobson JA, Saunders MM. Biomechanical analysis of in situ single versus double screw fixation in a nonreduced slipped capital femoral epiphysis model. J Pediatr Orthop. 2006;26:479–485. 18. Snyder RR, Williams JL, Schmidt TL, et al. Torsional strength of double- versus single-screw fixation in a pig model of unstable slipped capital femoral epiphysis. J Pediatr Orthop. 2006;26:295–299. 19. Early SD, Hedman TP, Reynolds RAK. Biomechanical analysis of compression screw fixation versus standard in situ pinning in slipped capital femoral epiphysis. J Pediatr Orthop. 2001;21:183–188. 20. Rab GT. The geometry of slipped capital femoral epiphysis: implications for movement, impingement, and corrective osteotomy. J Pediatr Orthop. 1999;19:419–424. 21. Carney BT, Weinstein SL, Noble J. Long-term follow-up of slipped capital femoral epiphysis. J Bone Joint Surg Am. 1991;73:667–674. 22. Leunig M, Casillas MM, Hamlet M, et al. Slipped capital femoral epiphysis: early mechanical damage to the acetabular cartilage by a prominent femoral metaphysis. Acta Orthop Scand. 2000;71:370–375. 23. Leunig M, Horowitz K, Manner H, et al. In situ pinning with arthroscopic osteoplasty for mild SCFE: a preliminary technical report. Clin Orthop Relat Res. 2010;468:3160–3167. 24. Walters R, Simon SR. Joint destruction: a sequel of unrecognized pin penetration in patients with slipped capital femoral epiphysis. In: Riley LH Jr, ed. The Hip Proceedings of the Eighth Open Scientific Meeting of the Hip Society. St Louis, MO: CV Mosby Company; 1980:145–164. 25. Jofe MH, Lehman W, Ehrlich MG. Chondrolysis following slipped capital femoral epiphysis. J Pediatr Orthop B. 2004;13:29–31.
Copyright
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2014 Wolters Kluwer Health, Inc. All rights reserved.
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
Biomechanical Comparison of Perpendicular Versus Oblique In Situ Screw Fixation of Slipped Capital Femoral Epiphysis Michael K. Merz, MD,* Farid Amirouche, PhD,* Giovanni F. Solitro, PhD,* Jeffrey A. Silverstein, MD,w Tyler Surma, BS,* and Prasad V. Gourineni, MD*z
Background: Percutaneous in situ single screw fixation is the preferred treatment for stable and unstable slipped capital femoral epiphysis (SCFE). The recommended screw placement is in the center of the epiphysis and perpendicular to the physis, which necessitates an anterior starting point for most SCFEs. A recent clinical study has shown good clinical results with a laterally based screw for SCFE, which is oblique to the physis. We sought to biomechanically compare these 2 techniques for load to failure and hypothesized that the laterally based oblique screw is equivalent or superior to an anteriorly based perpendicular screw. Methods: Twenty-two paired immature porcine femurs were used to compare the techniques. A SCFE model was created in all femurs using a previously published technique by performing a 30-degree posterior closing wedge osteotomy through the proximal physis. In the control group, a screw was placed perpendicular to the slip with an anterior starting point. In the experimental group, the screw was started as close to the midlateral cortex of the proximal femur as possible while maintaining the screw anterior to the posterior cortex of the femoral neck and ending at the apex of the epiphysis ignoring the resultant angle to the physis for the experimental group. The specimens were then potted and loaded in a physiologically relevant posteroinferior direction (30 degrees posterior from vertical) to determine load to failure (N) and stiffness (N/mm). Results: No statistical difference was found between the 2 groups in maximum load to failure or stiffness (P > 0.05). Conclusions: A laterally based screw oblique to the physis for in situ fixation in mild SCFE is not significantly different than an anteriorly based screw perpendicular to the physis in load to failure and stiffness in our study model. Clinical Relevance: In light of no difference in load to failure of these 2 constructs, surgeons may be more comfortable with the traditional lateral entry point while still aiming for screw placement in the center of head.
From the *Department of Orthopaedics, University of Illinois at Chicago, Chicago, IL; wSarasota Orthopedic Associates, Sarasota; and zAdvocate Children’s Hospital, Oak Lawn, IL. A Synthes educational grant donated all screws used in this study. The authors declare no conflicts of interest. Reprints: Michael K. Merz, MD, 835 S. Wolcott Avenue, M/C 844, Rm E270, Chicago, IL 60612. E-mail: [email protected]. Copyright r 2014 Wolters Kluwer Health, Inc. All rights reserved.
816 | www.pedorthopaedics.com
Key Words: slipped capital femoral epiphysis, pinning, biomechanical (J Pediatr Orthop 2015;35:816–820)
S
lipped capital femoral epiphysis (SCFE) is a common adolescent hip disorder with an incidence of 10.8 per 100,000 children in the United States.1 Approximately 20% of patients presenting with SCFE have bilateral involvement and another 20% to 40% are likely to progress to bilateral involvement.2,3 This accounts for a significant surgical burden of this disease worldwide. The gold standard for treatment of stable and unstable types of SCFE as defined by Loder et al4 is with percutaneous in situ screw fixation to prevent slip progression and to obtain physeal closure.5–7 Southwick angle has also been used to delineate mild (50 degrees) slips in SCFE.8 A mild slip as defined by Southwick is treated with a single in situ percutaneous screw. The current recommendation is to place a single screw into the center of the femoral head and perpendicular to the physis.9 Eccentric screw placement was shown to delay physeal closure6 and allow progression of the slip.5 A recently published case series10 described the results of 36 stable SCFE hips, which were treated with a lateral screw oblique to the physis instead of the standard anterior and perpendicular screw. No screw-related complications were noted and an adequate number of screw threads were able to fit proximal to the physis. The lateral technique is described as technically less demanding, which decreases the risk of fracture due to multiple passes with the drill11 and impingement of the screw head.12 In addition, it was theorized that the lateral starting point has stronger bone than the anteriorly based screw, which would decrease the risk of neck plow from a “windshield wiper” effect as described by Upasani et al13 and ultimately slip progression. We sought to create a biomechanical model to test the laterally based oblique screw compared with the gold standard anteriorly based perpendicular screw. We hypothesize that the lateral screw has superior load to failure than the anterior screw. If clinically equivalent or superior, the lateral technique of screw placement could J Pediatr Orthop
Volume 35, Number 8, December 2015
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
J Pediatr Orthop
Volume 35, Number 8, December 2015
offer a safer and less challenging way for orthopaedic surgeons to treat mild stable SCFE patients. Moreover, the similarity of the lateral technique to that of percutaneous pinning for femoral neck fractures in adults may make this technique more straightforward for orthopaedic surgeons who do not have a pediatric orthopaedic specialist to treat these patients in their area.
METHODS An initial pilot study was performed using 22 adult left femur Sawbones (Pacific Research Laboratories Inc., Vashon Island, WA). An Instron (Instron, Norwood, MA) loading machine was used to place an inferiorly directed force to failure on the specimens with either a point load using a sphere or a flat loading surface. Failure was defined as 10 mm of displacement of the femoral head. We repeated the experiment using immature porcine femora as described previously.13–15 Twenty-two skeletally immature (11 to 12 wk old), fresh, female porcine femora were obtained from 11 specimens. Previous work by Speer16 has shown the mechanical strength of the porcine bone is similar to an adolescent human. Soft tissues were stripped from the specimens and a posterior 30-degree angular wedge was resected from the femoral neck to create a mild SCFE. Each paired set of femora were tested against each other alternating the type of screw placed on each side. We initially used an opening wedge SCFE model in the Sawbones trial as it more closely resembles the pathology in SCFE, but as the closing wedge model is the most commonly published,13–15 we felt that using a closing wedge would have more results to compare against as an additional validation of our model and results. A guidewire was placed in a retrograde manner perpendicular to the physeal cut with an anterior starting point in the control group (Fig. 1). The accuracy of the placement was verified with fluoroscopy and measurement on anterior and frog-leg lateral views before screw insertion. A Synthes (Johnson & Johnson, West Chester, PA) 7.3 mm stainless steel cannulated screw with 32 mm partial threads was used for all specimens. The screw was advanced to the subchondral bone of the femoral head, which was verified both grossly and fluoroscopically. The laterally based oblique screws were inserted in a similar manner (Fig. 2). The starting point was on the lateral cortex proximal to the lesser trochanter aiming at the center of the head. Likewise, the screw was advanced to, but not through, the subchondral bone and verified grossly and fluoroscopically. Of note, only 1 length of screw (60 mm) was available for all specimens and therefore the head of the screw was proud on all specimens. This was not thought to affect the strength of the construct because it was the same for all specimens and compression of the physis has been disproven as an important aspect of fixation for SCFE.13 All specimens were potted in plaster of Paris distal to the lesser trochanter tilting 30 degrees posterior from the vertical axis. These were placed in an Instron loading Copyright
r
2014 Wolters Kluwer Health, Inc. All rights reserved.
Percutaneous In Situ Screw Fixation for SCFE
machine with a directly inferior direction at 5 mm/min instead of using 30 mm/min as previously described13–15 to more precisely observe the behavior for the samples. Similar to other studies, failure was defined as displacement of the femoral head of 10 mm.13–15 Maximum load to failure (N) and forces (N) were recorded every 100 ms at 2 N increment up to 8 N using the Merlin software (Instron). Stiffness (N/mm) was also determined using the computed average slope of the load displacement curve. Mean and SD were calculated for each group and compared using a 2-tailed Student t test.
RESULTS For the Sawbones trial, cantilever bending was noticed in the diaphysis during loading, particularly when the point load was applied. All Sawbones specimens were potted with approximately 5 cm of diaphysis exposed distal to the lesser trochanter. To eliminate cantilever bending in the porcine model, all specimens were potted up to the lesser trochanter. For the porcine model, the perpendicular screw construct had an average maximum failure load of 989.4 ± 339.3 N, and the oblique screw construct had an average maximum failure load of 916.2 ± 271.1 N. The maximum load to failure was not statistically significant between screw orientations (P = 0.62). The average forces at 2.5 mm of displacement were 386.8 ± 75.7 and 407.9 ± 69.3 N for the perpendicular and oblique constructs, respectively (P = 0.55). At 5 mm of displacement the average forces were 619.4 ± 194.2 and 640.3 ± 131.6 N for the perpendicular and oblique constructs, respectively (P = 0.79). The stiffness was estimated using the linear regression intercepting the origin of the obtained average curves, which had no significant difference: 107.8 N/mm (R2 = 0.94) for the perpendicular and 108.2 N/mm (R2 = 0.85) for the oblique screw construct. Without imposing origin interception the linear regression had higher coefficient of determination with values of stiffness of same order: 92.8 N/mm (R2 = 0.98) for the perpendicular and 85.9 (R2 = 0.94) for the oblique in situ fixation. Furthermore, the slope that best represents the tangent to the regression line not passing by the origin had: (load = 101.6+92.8 displacement) for perpendicular and (load = 151.9+85.9 displacement) for oblique. Graphical representation comparing displacement and load for the 2 models and their SDs is shown in Figure 3. The primary mode of failure for all specimens was due to head plow for both constructs. Head plow has been described previously as failure of fixation from movement of the femoral head inferiorly over a stable screw parallel to the physis, and is one of the most common modes of construct failure.13 No specimens failed due to fracture of the femoral head or neck.
DISCUSSION This study compared the effect of the direction of screw placement on fixation strength for in situ fixation www.pedorthopaedics.com |
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
817
J Pediatr Orthop
Merz et al
Volume 35, Number 8, December 2015
FIGURE 1. Anterior (A) and lateral (B) view of porcine model with screw starting anteriorly and oriented perpendicular to the physis.
of a mild SCFE using a porcine model. To the best of our knowledge, no biomechanical study has compared these techniques using physiological loads. Previous clinical
studies have established that single screw fixation is both efficacious and safer in treatment of SCFE than 2 screws.6,7
FIGURE 2. Anterior (A) and lateral (B) view of porcine model with screw starting laterally and oriented oblique to the physis.
818 | www.pedorthopaedics.com
Copyright
r
2014 Wolters Kluwer Health, Inc. All rights reserved.
Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
J Pediatr Orthop
Volume 35, Number 8, December 2015
Biomechanical Comparison of Screw Fixations 1200 Oblique in situ screw fixation Perpendicular in situ screw fixation
1000 Applied Load [N]
Standard deviation for Oblique Standard deviation for Perpendicular
800 600 400 200 0 0
2
4
6
8
10
Displacement [mm]
FIGURE 3. Graph comparing fixation strength of the porcine models between perpendicular and oblique screw fixation including SD.
Multiple biomechanical studies have evaluated different techniques for in situ fixation of SCFE. They have shown increased load to failure14,17 and increased torsional strength17,18 using 2 screws compared with 1 screw; however, all studies caution against using 2 screws due to the danger of penetration into the joint. A previous study19 recommended compression across the physis with screw fixation because of a 47% increase in stiffness, but did not show differences in load to failure between compression and noncompression groups. A subsequent study13 using a porcine model under physiological loads showed a statistically significant greater load to failure and stiffness for screws placed with an even number of threads on both sides of the physis compared with more threads across the physis. Fully versus partially threaded screws have also been compared and ultimately found to have no significant difference in load to failure.15 However, in situ screw fixation does not address the residual deformity of the proximal femur. The residual deformity can cause hip impingement20,21 and arthritis in the long term, especially in the more severe slips.22 Similarly, in Gourineni’s10 study, the laterally based oblique screw was initially used because it was noticed that the residual deformity of in situ fixation of SCFE caused impingement and arthritis in more severe slips. As a result, the technique was modified to allow concurrent resection of the anterior femoral neck prominence without risking screw failure which has been described by Leunig et al.23 In this study we endeavored to determine whether there is a difference between a lateral screw oblique to the physis compared with an anterior perpendicular screw for maximum load to failure and stiffness. Our study showed no significant difference between these 2 techniques for in situ fixation of a mild SCFE in the porcine model. We used an immature porcine model previously described with a similar number of specimens for each treatment group.13–15,17,18 Our values for maximum load to failure and stiffness are comparable with those for a single-crew construct in previous studies,13–15 and therefore we feel our model was appropriate. For the porcine model, we Copyright
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Percutaneous In Situ Screw Fixation for SCFE
chose only to use a distributed load instead of the point load because the distributed load more closely represents the conformity between the acetabulum and femoral head. Walters and Simon24 described a “blind spot” radiographically on the head of the femur that can lead to unrecognized screw penetration and ultimately chondrolysis with perpendicularly oriented screws. Jofe et al25 described their experience with 17 patients who had chondrolysis after in situ fixation of a SCFE and found that 15 (88%) patients had obvious signs of intra-articular penetration with the remaining 2 patients’ screws within 2 mm of the articular surface on radiographs. These 2 studies highlight the relation between screw penetration and chondrolysis as well as the difficulty in intraoperatively determining whether or not screw penetration has occurred. We theorize that using a laterally based screw may have less intra-articular penetration due to the perpendicular orientation of the screw to an anteroposterior radiograph taken intraoperatively. This would avoid the “blind spot” and make any penetration more readily apparent on radiographs. Our study has several limitations. It is a cadaveric animal model treated in vitro attempting to reproduce an in vivo condition. Porcine femora have been shown to be suitable for SCFE models, but there are inherent differences in size and morphology compared with an adolescent human. Having matched pairs of femora from the same animal reduces the amount of heterogeneity between specimens. Similarly, all specimens were of the same sex and within 1 week of the same age at the time of sacrifice. Another limitation is that the load to failure does not represent the cyclical loading of the hip experienced in vivo. A single screw length available for all specimens also may have affected load to failure particularly in the perpendicular screws. There was a greater length of screw in the femoral bone for the lateral screw models. We initially hypothesized that the lateral screw may be superior to the anterior screw partially due to this difference in screw length. The longer screw with a longer lever arm and support in the bone may lead to superior loads to failure in the lateral group. In our study though there was no significant difference between the 2. Potentially in a larger model, the longer lever arm would have more of an effect. It was noted when the femurs were sectioned after loading that some perpendicular specimens had threads engaging the anterior cortex of the femoral neck which was not seen in any of the femurs with oblique screws. This may have created a stronger construct for the perpendicular screw due to more cortical fixation of the screw. However, we do not consider the threads engaging the anterior cortex to influence plow in the anterior screw models for this study. Had the lateral screw models failed due to neck plow instead of head plow, we may have considered this as a difference between the specimens. In addition, all specimens of both groups failed due to head plow. The anterior screw models with threads engaging the anterior femoral neck cortex did not statistically significantly affect load to failure. www.pedorthopaedics.com |
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CONCLUSIONS Laterally based oblique in situ screw fixation of stable SCFE is not significantly different in load to failure and stiffness compared with an anteriorly based perpendicular screw. In light of a recent clinical series showing no complications with this technique, it is a reasonable alternative to a perpendicularly placed screw. It may have the additional benefit of decreased screw penetration because it is outside the “blind spot” and perpendicularly oriented to an anteroposterior radiograph intraoperatively. In addition, it is a familiar technique due to its similarity to percutaneous pinning of adult femoral neck fractures for the general orthopaedic surgeon who may not have a pediatric orthopaedic specialist to treat these patients in their area. REFERENCES 1. Lehmann CL, Arons RR, Loder RT, et al. The epidemiology of slipped capital femoral epiphysis: an update. J Pediatr Orthop. 2006;26:286–290. 2. Riad J, Bajelidze G, Gabos PG. Bilateral slipped capital femoral epiphysis: predictive factors for contralateral slip. J Pediatr Orthop. 2007;27:411–414. 3. Hagglund G, Hansson LI, Ordeberg G, et al. Bilaterality in slipped upper femoral epiphysis. J Bone Joint Surg Br. 1988;70:179–181. 4. Loder RT, Richards BS, Shapiro PS, et al. Acute slipped capital femoral epiphysis: the importance of physeal stability. J Bone Joint Surg Am. 1993;75:1134–1140. 5. Aronson DD, Carlson WE. Slipped capital femoral epiphysis. A prospective study of fixation with a single screw. J Bone Joint Surg Am. 1992;74:810–819. 6. Ward WT, Stefko J, Wood KB, et al. Fixation with a single screw for slipped capital femoral epiphysis. J Bone Joint Surg Am. 1992;74: 799–809. 7. Carney BT, Birbaum P, Minter C. Slip progression after in situ single screw fixation for stable slipped capital femoral epiphysis. J Pediatr Orthop. 2003;23:584–589. 8. Southwick WO. Osteotomy through the lesser trochanter for slipped capital femoral epiphysis. J Bone Joint Surg Am. 1967;49:807–835. 9. Herring JA. Tachdjian’s Pediatric Orthopaedics. 4th ed. Philadelphia, PA: Saunders Elsevier; 2008:853–854. 10. Gourineni P. Oblique in situ screw fixation of stable slipped capital femoral epiphysis. J Pediatr Orthop. 2013;33:135–138.
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11. Canale ST, Azar F, Young J, et al. Subtrochanteric fracture after fixation of slipped capital femoral epiphysis: a complication of unused drill holes. J Pediatr Orthop. 1994;14:623–626. 12. Goodwin RC, Mahar AT, Oswald TS, et al. Screw head impingement after in situ fixation in moderate and severe slipped capital femoral epiphysis. J Pediatr Orthop. 2007;27:319–325. 13. Upasani V, Kishan S, Oka R, et al. Biomechanical analysis of single screw fixation for slipped capital femoral epiphysis: are more threads across the physis necessary for stability? J Pediatr Orthop. 2006;26:474–478. 14. Kishan S, Upsani V, Mahar A, et al. Biomechanical stability of single-screw vs. two-screw fixation of an unstable slipped capital femoral epiphysis model: effect of screw position in the femoral neck. J Pediatr Orthop. 2006;26:601–605. 15. Miyanji F, Mahar A, Oka R, et al. Biomechanical comparison of fully and partially threaded screws for fixation of slipped capital femoral epiphysis. J Pediatr Orthop. 2008;28:49–52. 16. Speer DP. The John Charnley Award Paper. Experimental epiphysiolysis: etiologic models slipped capital femoral epiphysis. Hip. 1982:68–88. 17. Segal LS, Jacobson JA, Saunders MM. Biomechanical analysis of in situ single versus double screw fixation in a nonreduced slipped capital femoral epiphysis model. J Pediatr Orthop. 2006;26:479–485. 18. Snyder RR, Williams JL, Schmidt TL, et al. Torsional strength of double- versus single-screw fixation in a pig model of unstable slipped capital femoral epiphysis. J Pediatr Orthop. 2006;26:295–299. 19. Early SD, Hedman TP, Reynolds RAK. Biomechanical analysis of compression screw fixation versus standard in situ pinning in slipped capital femoral epiphysis. J Pediatr Orthop. 2001;21:183–188. 20. Rab GT. The geometry of slipped capital femoral epiphysis: implications for movement, impingement, and corrective osteotomy. J Pediatr Orthop. 1999;19:419–424. 21. Carney BT, Weinstein SL, Noble J. Long-term follow-up of slipped capital femoral epiphysis. J Bone Joint Surg Am. 1991;73:667–674. 22. Leunig M, Casillas MM, Hamlet M, et al. Slipped capital femoral epiphysis: early mechanical damage to the acetabular cartilage by a prominent femoral metaphysis. Acta Orthop Scand. 2000;71:370–375. 23. Leunig M, Horowitz K, Manner H, et al. In situ pinning with arthroscopic osteoplasty for mild SCFE: a preliminary technical report. Clin Orthop Relat Res. 2010;468:3160–3167. 24. Walters R, Simon SR. Joint destruction: a sequel of unrecognized pin penetration in patients with slipped capital femoral epiphysis. In: Riley LH Jr, ed. The Hip Proceedings of the Eighth Open Scientific Meeting of the Hip Society. St Louis, MO: CV Mosby Company; 1980:145–164. 25. Jofe MH, Lehman W, Ehrlich MG. Chondrolysis following slipped capital femoral epiphysis. J Pediatr Orthop B. 2004;13:29–31.
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Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
