Avascular Necrosis of the Femoral Head After Osteosynthesis of Femoral Neck Fracture

Abstract

The reported incidence of avascular necrosis after femoral neck fracture fixation varies widely, and there is no consensus regarding its risk factors. We evaluated the incidence of avascular necrosis of the femoral head with the use of contemporary techniques for femoral neck fracture fixation. We then sought to determine what potential risk factors influenced the development of avascular necrosis.

Between 1990 and 2005, one hundred sixty-three intracapsular femoral neck fractures in 163 patients were treated with internal fixation at our level-I trauma center. All patients were monitored until conversion to total hip arthroplasty (THA) or for a minimum of 2 years postoperatively. Ten patients (10 hips) died and 7 patients (7 hips) were lost to follow-up. The remaining 146 patients (146 hips) had a mean 5.2 years of follow-up (range, 3 months to 17 years). The incidence of avascular necrosis was 25.3% (37 hips). The average time to diagnosis of avascular necrosis was 18.8 months (range, 3-47 months). Patient sex, age, interval from injury to surgery, and mechanism of injury were statistically not associated with the development of avascular necrosis. The quality of fracture reduction, adequacy of fixation, degree of displacement, and comminution of the posterior cortex were significantly associated. After we controlled for patient and radiographic characteristics, multivariate analyses indicated that the important predictors for avascular necrosis are poor reduction (odds ratio=13.889) and initial displacement of the fracture (odds ratio=4.693).

Internal fixation is the treatment of choice for femoral neck fracture in young adults.¹ However, it has a relatively high rate of failure from loss of fixation, nonunion, and avascular necrosis of the femoral head.^1,2 Avascular necrosis causes a repaired femoral neck fracture to fail and may make additional surgery necessary.^3-5 However, the reported incidence of avascular necrosis after femoral neck fracture fixation varies between 7% and 80%,^1,6-10 and no consensus exists regarding the risk factors of avascular necrosis of the femoral head after internal fixation of femoral neck fracture.

This study evaluates the incidence of avascular necrosis with the use of contemporary techniques for femoral neck fracture fixation and investigates the potential risk factors influencing the development of avascular necrosis.

Materials and Methods

We operatively treated 554 intracapsular femoral neck fractures in 522 consecutive patients between 1990 and 2005. Of the 522 patients, we excluded those treated by arthroplasty (324 hips [58.5%]) and those with pathologic fracture due to neoplasm (22 hips [4.0%]), an associated femoral shaft fracture (33 hips [6.0%]), preexisting hip disease (6 hips [1.1%]), heavy alcohol abuse (4 hips [0.7%]), and prednisolone use within the preceding 3 years (2 hips [0.3%]). A treatment protocol was adopted whereby patients younger than 70 years were treated with internal fixation. This left 163 patients (29.4%) with 163 intracapsular femoral neck fractures included in this study.

The patients were monitored prospectively by means of scheduled visits to determine whether avascular necrosis had developed, as defined by the criteria of Ficat.¹¹ Diagnosis of avascular necrosis was done by radiographs. All patients gave informed consent for participation, and our Institutional Review Board approved the protocol. All patients were monitored until conversion to total hip arthroplasty (THA) or for a minimum of 2 years. Ten patients (10 hips) died from causes unrelated to surgery, and 7 patients (7 hips) were lost to follow-up before the end of the minimum 2-year follow-up period; this left 146 patients (146 hips) in the study. None of the 17 patients (10.4%) who died or were lost to follow-up showed avascular necrosis by the time of their final evaluation (8-23 months after initial surgery). The study group comprised 83 men and 63 women with a mean age of 45.7 years (range, 17-70 years). Mean duration of follow-up was 5.2 years (range, 3 months to 7 years) because 7 patients required conversion to THA after <2 years of follow-up. Therefore, 139 patients were monitored clinically and radiographically for a minimum of 2 years (mean, 5.6 years; range, 2-7 years).

In patients with polytrauma, life-threatening injuries were treated first and then femoral neck fractures were treated urgently. The average time elapsed between injury and surgical treatment was 3.4 days (range, 4 hours to 23 days). Fifty-one fractures (34.9%) were treated within 12 hours of injury (mean, 8.2 hours; range, 4-12 hours), and 95 fractures (65.1%) were treated >12 hours after injury (mean, 4.9 days; range, 15 hours to 23 days) because of late presentation or the need to treat other life-threatening injuries or diseases. Fifty-five fractures (38%) were nondisplaced and 91 (62%) were displaced.

All operations were performed by 1 surgeon (B.W.M.). Patients with undisplaced fractures were stabilized in situ. Patients with displaced fractures to be treated with internal fixation were placed on the fracture table, and closed reduction was attempted initially with the hip in extension, using minimal traction combined with internal rotation, as suggested by Garden.¹² A capsulotomy was not performed when a closed reduction was possible. There were 9 open reductions with capsulotomies. All fractures were fixed with either a sliding hip screw (57 hips) or cannulated screws (89 hips). Screw fixation was achieved with three or four 6.5-mm cannulated, partially threaded, cancellous screws. All patients were allowed progressive partial weight bearing as tolerated on postoperative day 3.

Because there is no generally accepted method of grading the quality of reduction and fixation, we graded reduction and fixation based on the degree of residual gap and angulation.^4,12-14 Reduction was considered satisfactory if the Garden alignment index was 160° to 180° as measured on anteroposterior radiograph and 180° on lateral radiograph, with a gap of <5 mm. In hips with a sliding screw, satisfactory fixation was defined as center-to-center or inferior-to-posterior positioning of the lag screw; in hips with cannulated screws, it was defined as parallel positioning of screws, no penetration into the joint of the screw, and screw insertion in a triangular pattern. Nonunion was defined as fixation failure, loss of reduction, or persistent visible fracture line at a minimum of 12 months after the index procedure.¹⁵ At final follow-up, each patient was evaluated by the treating surgeon for pain, walking ability, and the need for walking aids. Radiographs of all patients were analyzed by 1 reader who was blinded to the patients’ clinical and radiographic histories.

Statistical analysis to identify factors predisposing to avascular necrosis was performed with SAS software version 9.13 (SAS Institute, Cary, North Carolina). Independent samples test was used for comparisons of mean age, and chi-square test was used for analyses of sex, time interval from injury to surgery, mechanism of injury, fixation method, displacement, osteoporosis, comminution, quality of reduction, and fixation status. We defined osteoporosis as a Singh index of <3.¹⁶ The level of significance was set at P<.05. Potential predictive variables, including radiographic and clinical factors, were also analyzed in a multivariate logistic regression analysis to determine if they influenced the development of avascular necrosis. With conversion to THA as the endpoint, survival statistics were calculated and plotted according to the Kaplan-Meier method.

Results

The incidence of avascular necrosis was 25.3% (37/146). Avascular necrosis was classified as stage II in 7 hips, stage III in 17, and stage IV in 13. Five fractures were associated with the development of both avascular necrosis and nonunion. Other complications included nonunion in 4 hips (2.7%). The average elapsed time until diagnosis of avascular necrosis after surgery was 18.8 months (range, 3-47 months). At last follow-up, 23 of the 146 fractures (16%) had been treated with THA because of the development of avascular necrosis. The average elapsed time from index surgery to conversion to THA was 3.3 years (range, 3 months to 8.3 years). With conversion to THA as the endpoint, the cumulative survival rates for the native femoral head were 84.4% (95% confidence interval [CI], 75.8%-90.1%) at 5 years and 73.0% (95% CI, 59.7%-82.5%) at 10 years (Figure). A total of 9 patients (6%) required procedures in addition to THA.

Figure: Kaplan-Meier survivorship curve demonstrating survival of the femoral head without conversion to THA. Error bars show 95% confidence intervals.

Patient sex, age, time interval from injury to surgery, the method of reduction, and mechanism of injury were statistically not associated with the development of avascular necrosis. There was no difference between the types of fixation methods (dynamic hip screw vs multiple screws; P=.829). Singh indices were also not different between groups. Other radiographic parameters, including quality of fracture reduction, adequacy of fixation, degree of displacement, and comminution of the posterior cortex, were significantly different (Table 1). The rate of avascular necrosis was different between displaced and undisplaced fracture groups. Only 4 of 55 patients (7.3%) with displaced fractures were found to have avascular necrosis at follow-up (P=.000). After we controlled for patient and radiographic characteristics, multivariate analyses indicated that the significant predictors of avascular necrosis were poor reduction (odds ratio=13.889; 95% CI, 1.972-100.000) and initial displacement of the fracture (odds ratio=4.693; 95% CI, 1.362-16.178).

Three patients experienced intraoperative complications. In 2 patients, penetration of an intra-articular screw necessitated a return to the operating room for screw exchange, and in 1 patient, a broken guide wire was left in place. At last follow-up, 123 patients with a preserved femoral head had excellent clinical function: 119 patients (97%) had no or slight pain (associated with no activity restriction and no use of analgesics), and 4 had moderate pain (associated with slight activity restriction and regular use of non-narcotic analgesics). Most of the patients did not use walking aids; only 3 used a cane occasionally. Among the 14 patients with avascular necrosis who had not undergone conversion to THA, 10 patients had slight pain and 4 patients had moderate pain with intermittent cane use.

Discussion

The reported incidence of avascular necrosis after femoral neck fracture fixation varies widely, and no consensus exists regarding the factors predicting avascular necrosis, although considerable evidence relates risk factors (eg, the time interval from initial injury to surgery, accuracy of reduction, fixation status, and posterior cortical comminution) to development of avascular necrosis in internally fixed femoral neck fractures. We therefore wanted to determine whether a number of radiographic and clinical risk factors influenced the development of avascular necrosis and, if so, for how long after the occurrence of avascular necrosis and initial fixation of femoral neck fractures.

Our study has several limitations. The number of patients involved was small and the period was long. A prospective clinical trial involving multiple centers would be a more practical way to garner adequate numbers of patients to detect the causes of avascular necrosis after femoral neck fracture, although the feasibility of such a trial is doubtful because other factors such as operating room availability would not necessarily be predictable or controllable. Diagnosis of avascular necrosis on radiographs, especially when hardware is in situ, can be challenging, although magnetic resonance imaging has also a limited role in evaluation of avascular necrosis with hardware in situ. Reliance on radiographs to diagnose avascular necrosis implies the possibility that no patients with Ficat stage I avascular necrosis were detected. That the diagnosis of avascular necrosis was not confirmed by another observer is an important factor in this study. This increased the risk of systemic bias, although it eliminated interobserver variation.

Femoral neck fractures are relatively rare fractures, and those in our study were treated over a long enough time period that surgical techniques and internal fixation devices evolved. We used screws or dynamic hip screws for fixation with no guiding principle, although there was no difference in the incidence of avascular necrosis between implants. We excluded patients with heavy alcohol abuse and those with prednisolone use within the 3 years preceding our study because these substances have been implicated in the development of avascular necrosis. The strengths of our study include the treatment of a large number of consecutive patients at a single institution and high rates of follow-up, which allowed us to accurately determine rates of avascular necrosis and femoral head survival for a relatively long duration.

The overall rate of avascular necrosis in our patients was 25.3%, which is consistent with rates reported for previous studies (Table 2).^5,13,17-21 Our study demonstrates that the use of contemporary fixation techniques for the treatment of femoral neck fractures is associated with a low nonunion rate (2.7%; 4/146). When 5 fractures associated with the development of both avascular necrosis and nonunion were considered, the nonunion rate was 6.2% (9/146). However, the use of contemporary methods for the treatment of femoral neck fracture in our study did not significantly decrease the development of avascular necrosis. Fortunately, a substantial number of our patients who had avascular necrosis did not require THA (62%; 23/37). Among the 14 patients with avascular necrosis who did not undergo conversion to THA, the femoral head was preserved in 5 patients with secondary procedures such as transtrochanteric rotational osteotomy (2 hips) and core decompression (3 hips). The remaining 9 patients reported slight pain and no activity restrictions.

The mean interval between internal fixation and diagnosis of avascular necrosis in this study was 18.8 months (range, 3-47 months). Although avascular necrosis usually develops within 3 years after fixation, it can occur after 3 years.¹⁷ Therefore, the incidence of avascular necrosis may be higher if all patients were monitored for a longer period. However, we monitored patients for a minimum of 2 years (mean, 5.2 years). We monitored 92 of 130 patients (71%) for >3 years; 16 patients developed avascular necrosis within <3 years of monitoring. In our study, all avascular necrosis after femoral neck fracture fixation was diagnosed within 4 years.

Several studies have attempted to identify factors predictive of avascular necrosis.^1,4,20,22 With the numbers available, we found no association between the development of avascular necrosis and patient age, sex, type of fixation, interval from injury to surgery, or mechanism of injury. In some studies,²³ age is been considered a modulating factor, although reported data are contradictory.^1,24 There is a tendency for avascular necrosis to be reported more often in women than in men.^1,25 We did not find the presence of osteoporosis to be a significant prognostic factor. We agree with other researchers²⁶ that the Singh index has wide intraobserver variation and poor reproducibility.

Although the effect of delaying surgery is debatable and the data are inconclusive, most studies have proposed early surgery.^27,28 Using the logistic regression model to evaluate the effect of factors, 1 group of researchers found that only time elapsed until fracture reduction significantly affected the development of avascular necrosis.²⁰ However, 2 large clinical trials reported no correlation between the time interval to surgery and the development of avascular necrosis.^1,7 In our study, the average time elapsed between injury and surgical treatment was 3.4 days because of associated medical problems; the rates of avascular necrosis among patients who underwent fixation within the first 12 hours of trauma did not differ from those of patients who underwent fixation later.

Fracture displacement is considered to be a main factor for the development of avascular necrosis. In a review of 23 series reported between 1930 and 1970, the average avascular necrosis rate was 8.5% for undisplaced fractures and 29.3% for displaced fractures.³ Other researchers have demonstrated a strong correlation between the development of avascular necrosis and fragment displacement.²⁹ Our study showed an avascular necrosis rate of 7.3% for undisplaced fractures and 33.9% for displaced fractures. Our findings support the premise that the quality of reduction has an impact on the development of avascular necrosis. The incidence of avascular necrosis was 20% in patients with satisfactory reduction and 75% in patients with unsatisfactory reduction, which was statistically significant (P=.000).

Conclusion

Even with the use of contemporary techniques for the treatment of femur neck fractures, we did not see a significant decrease the development of avascular necrosis. Our study findings confirm that most important predictors are initial displacement of the fracture and poor reduction. To reduce the incidence of avascular necrosis after femoral neck fracture, anatomic reduction is essential.

References

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Authors

Dr Min is from the Department of Orthopedic Surgery, and Dr Kim is from the Department of Emergency Medicine, School of Medicine, Keimyung University, Daegu, Korea.

Drs Min and Kim have no relevant financial relationships to disclose.

Correspondence should be addressed to: Byung-Woo Min, MD, Department of Orthopedic Surgery, Keimyung University Dongsan Medical Center, 194 Dongsan-dong, Joong-gu, Daegu 700-712, Korea (min@dsmc.or.kr).

doi: 10.3928/01477447-20110317-13