Critical analysis of factors determining mechanical failures in proximal femoral nailing

INTRODUCTION

Fractures of proximal femur are more common in elderly population with intertrochanteric fractures contributing nearly 50% of them.¹ Several fixation devices like sliding hip screw, intramedullary devices have been developed to overcome the difficulties encountered in the treatment of unstable intertrochanteric fractures. Studies have shown that the functional outcome of the patients with sliding hip screw in unstable intertrochanteric fractures is comparatively less due to excessive screw sliding resulting in higher incidence of screw cut-out.^2-4

Intramedullary nailing especially proximal femoral nailing has become a popular method of stabilization of unstable intertrochanteric fracture. Various studies have shown that PFN is a better implant as it provides support to posteromedial wall and resists excessive fracture collapse.^5-7 Near-anatomical reduction with slight valgus alignment and optimal positioning of lag screw and antirotation screw are of paramount importance for good functional outcome. Still there are some pitfalls as implant failure like screw cut out, Z effect, reverse Z effect does occur in PFN due to specific unbalanced biomechanical forces acting on implant around hip joint.⁸ There are various factors determining the functional outcome in patients operated with proximal femoral nail namely the quality of fracture reduction, fracture pattern, the degree of osteoporosis, the position of lag and antirotation screw in the femoral neck, tip apex distance (TAD) difference between these screws, postoperative rehabilitation etc. Among these the fracture reduction quality, position of lag screw centre and TAD difference were critically analysed and their influence on the mechanical failure rate and functional outcome were studied.

METHODS

This was a prospective study conducted at Government Royapettah hospital, Chennai from June 2014 to May 2018. After institutional ethical committee clearance, 72 patients (42 male, 30 female) with age more than 18 years with unstable intertrochanteric fracture (AO type 31A2, 31A3) were included in our study. Patients with age less than 18 years, pathological fractures and those with arthritic changes of hip joint were excluded from the study.

All procedures were performed with patients in the supine position on a fracture table under general or spinal anaesthesia. Routine closed reduction method including abduction, traction and internal rotation was performed to get fracture reduction and confirmed by fluoroscopy. In some cases, mini open reduction was done to achieve near anatomical reduction. Fixation was done with intramedullary nail (10-12 mm in diameter), a lag screw (8.0 mm diameter) and an antirotation screw (6.4 mm diameter). The nail was interlocked distally with one or 2 screws. Patients were advised isometric quadriceps exercises on the first day after surgery. Partial weight bearing was allowed on the 4^th week after surgery. Later full weight was allowed after assessing the radiological union in follow up x-ray.

Radiological measurement

In immediate postoperative period standard AP radiographs of the hip were obtained with both legs positioned to an internal rotation of 15⁰. The lateral radiographs were taken with the contra lateral hip flexed and abducted. The reduction quality was categorized as positive medial cortical support (PMCS), neutral position (NP), negative medial cortical support (NMCS).The tip–apex distance (TAD) was measured separately for lag screw and antirotation screw in antero posterior x ray and the difference was calculated. The position of lag screw centre in the femoral neck was also calculated.

In AP view, the quality of fracture reduction, were classified via the position of the medial cortex between the femoral head–neck fragment and the shaft as: (1) Positive medial cortex support (PMCS) were the medial cortex of the head–neck fragment is located a little bit superomedially to the medial cortex of the femoral shaft. (2) Neutral position (NP): the medial cortex of head–neck and the shaft fragment are anatomically placed. (3) Negative medial cortex support (NMCS): the head–neck fragment is displaced laterally to the upper medial edge of the femoral shaft fragment (Figure 1). In the lateral view the reduction quality was assessed using the anterior cortical continuity between the head-neck fragment and the shaft fragment.⁹

Apex is the point where the centre neck line crosses the femur head cortex. D1–the length of the head neck interface line. D2–the distance to the centre of lag screw. TAD_LS–tip apex distance of lag screw. TAD_AR–tip apex distance of antirotation screw.

For measurement of lag screw centre, the femoral neck and head were divided into four quadrants as following: first, the head neck transition points were marked where the head convex contour changed to neck concave contour. Then the head neck interface line (L₁) was drawn connecting these two points. Neck centre line (L₂) is drawn crossing the line L₁ in its centre (Figure 2). The head apex was defined as the point where the neck centre line crosses the femoral head cortical bone.¹ The measurements D₁ represents the length of L₁ and D₂ represents the distance between the centre axis of lag screw and inferior edge of L₁ (Figure 3).

The tip apex distance for the lag screw (TAD_LS) in antero posterior radiograph is defined as the distance in millimetre from the tip of lag screw to the apex of the femoral head. Similar the tip apex distance for antirotation screw (TAD_AR) was determined. The TAD difference is measured as TAD_AR –TAD_LS (Fig.3). Radiographs were taken for evaluation of fracture union and implant related complications at 3 and 6 months after surgery. In follow-up, two parameters to determine fracture impaction namely (1) The femoral neck–shaft angle (NSA), which is the angle between the two axes of head–neck and shaft medullary. (2) Femoral neck length (FNL), which is the distance between head centre and shaft medullary centre along the head–neck central axis.

Statistical analysis

The statistical analysis in our study was performed using SPSS software version 20.0 and the various factors influencing the mechanical failures (cut out, Z effect, reverse Z effect) like fracture reduction quality, position of lag screw centre, TAD difference were analysed using Pearson Chi-Square analysis. All data were analyzed at a significance level of p<0.05.

RESULTS

In our study, among 72 patients 42 were male, 30 were female patients with average age of 63 years. The most common mode of injury was slip and fall (72%) followed by road traffic accidents (24%). The average time interval between injury and surgery was 4 days (2 -8 days). The average follow up period in our study was 2 years.

Fracture consolidation was seen by mean period of 15.2 weeks (13-18 weeks). Among the cases 33% had excellent outcome, 42% of cases had good and 14% of cases had fair outcome. 11% of cases ended with poor outcome. We had 3 patients with screw cut-out, 3 patients with Z effect and 2 patients with reverse Z effect as mechanical failures. The quality of reduction had statistical significance (p=0.014) in the incidence of mechanical failure rate with 31% in patients with NMCS when compared to 6% and 4% in patients with NP and PMCS respectively (Figure 4). Also patients in NMCS group had significant loss in the mean neck–shaft angle and the femoral neck length compared to NP and PMCS group at the end 6 months. Moreover the amount of lateral screw slide at final follow up was more in NMCS (mean value=5.4 mm) when compared to NP (mean value=3.2 mm) and PMCS (mean value=2.5 mm) (Table 1).

PMCS- positive medial cortical support, NP- neutral position, NMCS- negative medial cortical support, NSA- neck shaft angle, FNL-femoral neck length.

The position of lag screw centre had no statistical significance (p=0.547) in the incidence of mechanical failure rate in our study, but the functional outcome was comparatively better in patients with the lag screw centre in the third quadrant (D₂/D₁ = 0.25-0.5) (Table 2 and 4).

The tip apex distance (TAD) difference between antirotation screw and lag screw was statistically significant in determining the mechanical failures with patients with TAD difference <10 mm having more failures than patients with TAD difference >15 mm (Table 3 and 5) (Figure 5).

TAD_LS–Tip apex distance of lag screw, TAD_AR–Tip apex distance of antirotation screw.

DISCUSSION

Unstable intertrochanteric fractures are always challenging to treat as it is associated with lot of complications. Intramedullary implants like gamma nail, proximal femoral nail, proximal femoral nail antirotation, are generally biomechanically more stable over their extramedullary counterparts and numerous studies have shown good results following their use.^10,11 The proximal femoral nail was devised by the AO/ASIF group in 1996 which includes two proximal screws namely lag screw and an antirotation screw.¹² However, complications with PFN usage includes screw cut-out, Z or reverse Z effect, and fracture of lateral trochanteric wall, fracture below the distal locking site.¹³ Fogagnolo et al in their study showed a complication rate of about 23.4% with the usage of PFN.¹⁴ Similarly Uzun et al showed complications like nonunion was seen in 5.7%, secondary varus collapse in 25.7%, screw cut-out in 5.7%.¹⁵

In view of various literatures the preoperative fracture reduction quality had always been prior to the positions of implants determining mechanical failures in unstable intertrochanteric fractures. Most of them recommend slight valgus position of the fracture fragments with posteromedial cortical continuity for successful outcome. Moreover the compression of the fracture fragments which is essential for fracture union can be achieved by intraoperative fracture compression by the surgeon and as well as the postsurgical compression provided by a fixation device with a sliding capability on weight bearing.⁹

The concept of non-anatomic positive cortex buttress reduction was first introduced by Gotfried, for displaced femoral neck fracture were the distal femoral neck fragment is positioned medially to the lower-medial edge of the proximal fracture fragment.^16,17 In our study we used the concept of positive medial cortical support which was first introduced by Shi-Min Chang et al in their study were the proximal fracture fragment was placed superomedial to the shaft segment.⁹ This type of reduction allows controlled fracture impaction by limited sliding of the head-neck fragment, thereby providing secondary axial and torsional stability between the head–neck fragment and the femur shaft.

In contrast, fracture collapse also termed uncontrolled fracture impaction, or excessive sliding was noted in patients with negative medial cortical support were the head–neck fragment is displaced laterally to the upper medial edge of the femoral shaft fragment. Excessive fracture collapse results in shortening of the abductor lever arm thereby resulting in permanent limping and increases morbidity. In the study by Shi-Min Chang et al, the mean loss of the femoral neck–shaft angle in PMCS, NP and NMCS groups was 0.7^o, 4.8^o, and 8.9^o, respectively at 3 months follow up and patients with PMCS had comparatively better functional outcome than NS and NMCS group.⁹ In our study, the mean loss of the femoral neck–shaft angle in PMCS, NP and NMCS groups was 3.1^o, 4.6^o, and 8^o respectively at 6 months follow up. Moreover the incidence of mechanical failures was more in NMCS (31%) when compared to NS (6%) and PMCS (4%).

Kyuzky et al in their biomechanical study on position of lag screw concluded that the placement of lag screw inferiorly in the AP plane maximizes biomechanical stiffness, and placement of the lag screw centrally in the lateral plane maximizes load to failure.¹⁸ Similarly in a study by Herman et al, the position of the lag screw centre in the third quadrant (D₂/D₁ = 0.25-0.5) which was defined as the safe zone, was the most important cofactor preventing the mechanical failure.¹ In our study there was no significant difference in the incidence of mechanical failure in relation to the position of lag screw centre, however it was comparatively less in patients with the lag screw centre in the third quadrant (D₂/D₁ = 0.25-0.5). As earlier highlighted by Aithala et al, it is a well known fact that the screw placement angle in proximal femoral nail is prefixed and hence unless proper preoperative neck shaft angle is achieved it is not possible to place the lag screw in the proper position.¹⁹

Baumgartner et al first described the concept of Tip Apex Distance (TAD). It is considered as one of the most important indicators determining the incidence of mechanical failure of both intramedullary and extramedullary devices. It is defined as the sum of the distance, from the tip of the lag screw to the apex of the femoral head, as measured on the antero posterior and lateral radiograph, in millimetre after correction has been made for magnification.^20,21 Previous studies recommend TAD of lag screw of less than 25 mm in order to avoid screw cut-out. Herman et al in his study, calculated both anteroposterior (AP) and axial components for both lag screw and antirotation screw and concluded that only the AP component of the TAD_LS, when checked separately was found to predict screw cut-outs or secondary reduction loss.¹

Kashigar et al first reported the clinical significance of calcar referenced tip apex distance (calTAD) as predictor for screw cut-out. They concluded that patients with calTAD less than 25 mm had low incidence of screw cut-out.²² In our study we studied the importance of tip apex distance (TAD) difference between antirotation screw and lag screw (TAD_AR-TAD_LS) in determining the functional outcome and mechanical failures. We found that the incidence of mechanical failures are more in patients with TAD difference less than 10 mm (n=6) than in patients with TAD difference more than 15 mm (n=0). Similarly Moihara et al also suggests the length of the antirotation screw to be shorter by atleast 15 mm otherwise it could take weight and can back out or migrate into joint leading to cut out.²³

Apart from these factors, the fracture pattern, osteoporotic index, preoperative mobility status, timing of surgery, associated injuries, post op mobilisation also plays a key role in determining functional outcome of patients operated with proximal femoral nail

CONCLUSION

From our study we like to conclude that the fracture reduction with positive medial cortical support with valgus alignment allows controlled sliding of the proximal fragment to contact with the femur shaft and achieves secondary stability, thereby decreasing the incidence of varus collapse and secondary reduction loss. Moreover larger tip apex distance difference (TAD_AR-TAD_LS) reduces the incidences of mechanical failures like screw cut out, Z- effect and reverse Z- effect.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: The study was approved by the institutional ethics committee

REFERENCES

1. 1. Herman A, Landau Y, Gutman G, Ougortsin V, Chechick A, Shazar N. Radiological evaluation of intertrochanteric fracture fixation by the proximal femoral nail. Injury. 2012;43(6):856–63.
2. 2. Kulkarni GS, Limaye R, Kulkarni M, Kulkarni S. Intertrochanteric fractures. Indian J Orthop. 2006;40:16–23.
3. 3. Gundle R, Gargan M.F, Simpson HRW. How to minimize failure of fixation of unstable intertrochanteric fractures. Injury. 1995;26(9):611-4.
4. 4. Simpson AH, Varty K, Dodd CA. Sliding hip screws: modes of failure. Injury. 1989;20(4):227–31.
5. 5. Baumgaertner MR, Curtin SL, Lindskog DM. Intramedullary versus extramedullary fixation for the treatment of intertrochanteric hip fractures. Clin Orthop Relat Res. 1998;348:87–94.
6. 6. Sadowski C, Lübbeke A, Saudan M, Riand N, Stern R, Hoffmeyer P. Treatment of reverse oblique and transverse intertrochanteric fractures with use of an intramedullary nail or a 95 degrees screw-plate:a prospective, randomized study. J Bone Joint Surg. Am. 2002;84(3):372–81.
7. 7. Park SR, Kang JS, Kim HS, Lee WH, Kim YH. Treatment of intertrochanteric fracture with the gamma AP locking nail or by a compression hip screw-a randomised prospective trial. Int Orthop. 1998;22(3):157–60.
8. 8. Gadegone WG and Salphale YS. Short proximal femoral nail fixation for trochanteric fractures. J Orthop Surg (Hong Kong). 2010;18(1):39-44.
9. 9. Chang S-M, Zhang Y-Q, Ma Z, Li Q, Dargel J et al. Fracture reduction with positive medial cortical support:a key element in stability reconstruction for the unstable pertrochanteric hip fractures. Arch Orthop Trauma Surg. 2015;135(6):811-8.
10. 10. Mainds CC, Newman RJ. Implant failures in patients with proximal fractures of the femur treated with a sliding screw device. Injury. 1989;20(2):98–109.
11. 11. Parker MJ, Handoll HH. Gamma and other cephalocondylic intramedullary nails versus extramedullary implants for extracapsular hip fractures in adults. Cochrane Database Syst Rev. 2008;3:CD93.
12. 12. Simmermacher RK, Bosch AM, Van der Werken C. The AO/ASIF- proximal femoral nail (PFN):a new device for the treatment of unstable proximal femur fractures. Injury. 1990;30(5):327–32.
13. 13. Rethnam U, Cordell-Smith J, Kumar TM, Simha A. Complex proximal femoral fractures in the elderly managed by reconstruction nailing–complications and outcomes:a retrospective analysis. J Trauma Manag Outcomes. 2007;1(7):1–7.
14. 14. Fogagnolo F, Kfuri M Jr, Paccola C. Intramedullary fixation of pertrochanteric hip fractures with the short AO-ASIF proximal femoral nail. Arch Orthop Trauma Surg. 2004;124:31–7.
15. 15. Uzun M, Erturer E, Ozturk I, Akman S, Seckin F, Ozcelik IB. Long term radiographic complications following treatment of unstable intertrochanteric femoral fractures with the proximal femoral nail and effects on functional results. Acta Orthop Traumatol Turc. 2009;43(6):457–63.
16. 16. Gotfried Y. The Gotfried (nonanatomic, closed) reduction of unstable subcapital femoral fractures. Tech Orthop. 2012;27(4):259–61.
17. 17. Gotfried Y, Kovalenko S, Fuchs D. Nonanatomical reduction of displaced subcapital femoral fractures (gotfried reduction). J Orthop Trauma. 2013;27(11):254–9.
18. 18. Kuzyk et al Femoral Head Lag Screw Position for Cephalomedullary Nails: A Biomechanical Analysis. J Orthop Trauma. 2012;26:414–21.
19. 19. Aithala JP, Rao S. Proximal Femoral Nailing: Technical Difficulties and Results in Trochanteric Fractures. Open J Orthop. 2013;3(5):234-42.
20. 20. Baumgaertner MR, Curtin SL, Lindskog DM. Intramedullary versus extra medullary fixation for the treatment of intertrochanteric hip fractures. Clin Orthop Relat Res. 1998;348:87–94.
21. 21. Baumgaertner MR, Curtin SL, Lindskog DM, Keggi JM. The Value of the Tip-Apex Distance in Predicting Failure of Fixation of Peritrochanteric Fractures of the Hip. J Bone Joint Surg Am. 1995;77(7):1058-64.
22. 22. Kashigar A, Vincent A, Gunton MJ, Backstein D, Safir O, Kuzyk PR. Predictors of failure for cephalomedullary nailing of proximal femoral fractures. Bone Joint J. 2014;96(8):1029–34.
23. 23. Morihara T, Arai Y, Tokugawa S, Fujita S, Chatani K, Kubo T. Proximal femoral nail for treatment of trochanteric femoral fractures. J Orthop Surg (Hong Kong). 2007;15:273–7.