The incidence of trochanteric fractures in elderly patients with osteoporosis is increasing dramatically, in proportion with the growth of the elderly population.2,14 Although newer methods of intramedullary fixation have been introduced, fixation with the dynamic hip screw (DHS) remains the treatment of choice for this type of fracture.1,3,4,6,9,11,12 Dynamic hip screw fixation is quick, straightforward, and uses controlled impaction during weight-bearing to stabilize the fracture, thereby facilitating healing.3,5,6,17,30 However, the incidence of fixation failure in elderly patients with osteoporosis who have trochanteric fractures fixed with DHS still must be considered. The most frequent mode of failure is lag screw cutout, which involves the collapse of the femoral neck shaft angle into varus, causing screw extrusion. Studies have shown that the incidence of lag screw cutout ranges from 5–23%, with a reoperation rate of 10%.3,4,6–9,13,20,22,27,29,31,33
Many factors are thought to contribute to lag screw cutout, such as bone quality, patient age, fracture pattern, quality and stability of the reduction, angle of the implant, and position of the lag screw within the femoral head.7,8,12,21 Several authors cite correct placement of the screw in the femoral head as the most important factor influencing lag screw stability.7–9,27,28 Although we agree that proper lag screw positioning is critical to a successful outcome, we question whether correct lag screw positioning can always be obtained. Because stability of the lag screw is difficult to achieve in mechanically weak bone, we wanted to determine if lag screw cutout incidence could be reduced by improving the purchase of the screw in the femoral head.
Lag screw purchase can be augmented with PMMA and calcium phosphate bone cements; however, these methods are technically-demanding, time consuming, and expensive.13 The drawbacks of fixation augmentation with PMMA include exothermic reaction, inability to be remodeled, and the risk of impaired healing if interposition occurs. The mechanical strength of calcium phosphate bone cements is questionable, the cost greater than PMMA, and their orthopaedic application has not been validated.
To optimize screw fixation and osteointegration, and strength, HA-coated AO/ASIF screws have been developed. In loaded and unloaded comparative animal studies, these screws surpassed similar standard screws in fixation strength and level of osteointegration.23–25
Our purpose was to do a well-controlled prospective randomized clinical study in patients with osteoporotic trochanteric fractures treated with DHS fixed with either standard AO/ASIF or HA-coated AO/ASIF lag and cortical screws. We wanted to compare the incidence of fixation failure and the quality of the clinical outcomes between the two groups.
MATERIALS AND METHODS
The study design was approved by our institute’s ethics committee. One hundred twenty consecutive women with osteoporosis with trochanteric fractures were selected. Patients then were checked against exclusion criteria that included a history of previous hip fracture, open fracture, fracture secondary to malignancy, hard or soft tissue infection at the fracture site, chemotherapy, or multiple fractures. As patients were admitted to the study, the objectives and randomization were explained and an informed consent was signed. Patients were randomized by a computer-generated list to receive 135° four-hole DHS fixed with either standard lag and cortical stainless steel AO/ASIF screws (Group A), or HA-coated lag and cortical AO/ASIF screws (Group B).
Group B screws were plasma-sprayed with HA in air (R. Mathys & Co, Bettlach, Switzerland). The crystallinity of the HA, calculated by x-ray spectroscopy, was greater than 70%. The HA coating was applied by plasma spray technique and was identical to that used in a previous study.10 The purity, calculated with mass spectroscopy, was greater than 97%. Calcium to phosphate ratio was 1.67 ± 0.01. The mean coating thickness was 56 ± 19 μm. Surface roughness was Ra = 5.5–8 μm.
Postoperatively, lag screw position was evaluated according to the zones described by Cleveland et al10 and used by Baumgaertner et al.7,8 Cleveland et al divided the femoral head into the following zones: superoanterior, superior-center, superoposterior, center-anterior, center-center, center-posterior, inferior-anterior, inferior-center and inferior-posterior. Positioning in any of the three superior zones was considered exclusion criteria for our study.
On discharge, follow-ups were scheduled at 1, 3, and 6 months. Patients who died before follow-up or who did not attend follow-ups were excluded from the study. The desired number of 60 participants for each group was achieved by replacing excluded patients with other patients who met the inclusion criteria.
Patients, Surgical Procedures, Outcome Assessment
The average age was 81 ± 8 years in the patients in Group A and 81 ± 6 years in the patients in Group B (not significant). In Group A, 42% of patients had A1 fractures and 58% of patients had A2 fractures. In Group B, 48% of patients had A1 fractures and 52% of patients had A2 fractures (not significant). The average BMD was 538 ± 105 in patients in Group A, and 568 ± 111 in patients in Group B (not significant). All fractures were fixed within 48 hours of injury. Cephalosporin was administered preoperatively (1 g intramuscularly) and continued every 8 hours for 72 hours. Patients received spinal anesthesia and were placed in a supine position on a radiolucent fracture table. Surgery was done by either the senior surgeon (AM), or the senior resident (CF) assisted by the senior surgeon (AM). A routine lateral approach to the proximal femur was used in all cases. A DHS was implanted using the technique described by Regazzoni et al.30 Both screws in Group A and Group B were implanted according to standard AO/ASIF implantation technique with AO/ASIF recommended instrumentation. Patients were mobilized within 24 hours of surgery. Full weight-bearing with a walker or crutches was encouraged as tolerated.
Postoperatively, tip-apex distance was measured,6–8 Baumgaertner et al7 reported that “... an accurate method for describing screw position and predicting cutout failure, tip-apex distance is the ...sum of the distance, in millimeters, from the tip of the lag screw to the apex of the femoral head, as measured on an anteroposterior radiograph and that distance as measured on a lateral radiograph, after correction has been made for magnification. The apex of the femoral head is defined as the point of intersection between the subchondral bone and a line in the center of and parallel to the femoral neck.”
The quality of fracture reduction, as seen on the immediate postoperative radiographs, was assessed as described by Baumgaertner and Solberg.8 “For a reduction to be considered good there are to be normal or valgus alignment on the anteroposterior radiographs, less than 20 degrees of angulation on the lateral radiograph and no more than four millimeters of displacement of any fragment. To be considered acceptable, a reduction had to meet the criterion of a good reduction with respect to either alignment or displacement, but not both. A poor reduction met neither criterion.”
In our study, lag screw cutout was defined as a variation in the femoral neck shaft angle greater than 10°, with extrusion of the lag screw from the femoral head exceeding 1 mm. Radiographic union was defined by the presence of trabeculae bridging the fracture site or obvious periosteal callus in the fracture line. The end point of the study was either a healed fracture at 6 months or the indication for revision.
In both groups, femoral neck shaft angle at 6 months was compared with the femoral neck shaft angle measured in the immediate postoperative radiographs. Patients who had cutout were not included in the 6-month evaluation of femoral neck shaft angle. Accordingly, fracture impaction was evaluated by the method described by Larsson et al,21 comparing the postoperative and end point radiographs.
Clinical results were assessed according to the Harris hip score and Short Form 36 at 6 months follow-up. Patients who had cutout were not included in the 6 months evaluation of clinical results.18,19
All continuous data are expressed in terms of mean and standard deviation of the mean. One-way analysis of variance (ANOVA) was done to test hypotheses about means of different groups. When the Levene test for homogeneity of variances was significant (p < 0.05), the Mann-Whitney test was used to check ANOVA results. Pearson’s chi square test, calculated by the Montecarlo method, was done to investigate the relationships between grouping variables. For all tests, p < 0.05 was considered significant. Statistical analysis was done by means of the Statistical Package for the Social Sciences (SPSS) software version 9.0 (SPSS Inc, Chicago, IL).
No differences in femoral head lag screw position in the Cleveland zones, postoperative femoral neck shaft angle, and quality of reduction were observed between the two groups (Figs 1, 2).
The average tip apex distance was 22 ± 4 in Group A and 23 ± 5 in Group B (not significant). No differences in the percentages indicating at risk for cutout (tip apex distance > 25 mm) or not at risk (tip apex distance ≤ 25 mm) were observed between the two groups (Fig 3). Lag screw cutout occurred in four patients in Group A, but not in Group B (p < 0.05; β = 0.8). All cutout cases occurred within three months after fixation.
Three patients had revision with bipolar prostheses; the fourth patient refused revision surgery (Fig 4). In accordance with the results of Baumgaertner et al, no patient in either group with a tip apex distance ≤ 25 mm had cutout.6 In the standard group, 11 patients with cutout had a tip apex distance > 25 mm. However, no patient in the HA-coated screw group having a tip apex distance > 25 mm had cutout.
In Group A, a reduction in the femoral neck shaft angle was observed with time, whereas in Group B there was no reduction at 6 months. Femoral neck shaft angle was greater (p = 0.003) in Group B than in Group A. In Group A, the average femoral neck shaft angle was 134° ± 5° postoperatively and 127° ± 12° at 6 months (p = 0.003). In Group B, the femoral neck-shaft angle was 134° ± 7° postoperatively and 133° ± 7° at 6 months (not significant).
Fracture impactions were 13 ± 15 mm in Group A and 10 ± 7 mm in Group B (not significant). At 6 months, the Harris hip scores were 63 ± 22 points in patients in Group A and 70 ± 18 points in patients in Group B (p = 0.02). Short Form-36 scores were 56 ± 24 in patients in Group A and 62 ± 20 in patients in Group B (not significant).
Fixation of osteoporotic bone is a challenge for the orthopaedic surgeon. Because of the poor bone quality, stable fixation can be difficult to achieve and functional outcomes can be poor. Considering that in any elderly hip fracture population a certain number of patients will have healthy bone, dual energy x-ray absorptiometry (DEXA) analysis was an inclusion criterion. To the best of our knowledge, our study is the first to analyze lag screw cutout incidence and bone quality, in a well-controlled population of patients with osteoporotic trochanteric fractures.
In the orthopaedic community, there is consensus regarding the importance of proper lag screw positioning in the femoral head, with several authors favoring a central or inferior position and considering an anterosuperior and posteroinferior position at risk for failure.6,8,11,27 Baumgaertner et al6,8 stated that the most important parameter for evaluating screw position is tip apex distance. In these studies, none of the screws with a tip apex distance ≤ 25 mm cut out, but there was a strong correlation between increasing tip apex distance and the rate of cutout. The results obtained in our study with the standard screws confirm the findings reported by Baumgaertner.6,8 None of the standard screws with tip apex distance ≤ 25 mm cut out. However, in the HA-coated screw group, none of the HA-coated screws with a tip apex distance > 25 mm cut out. Even in cases of less than optimal screw positioning in the femoral head, adequate fixation was obtained. We think that this superior fixation was achieved because of the osteointegrative properties of the coating.24–27
Dynamic hip screw fixation with HA-coated screws was shown to be superior to DHS fixation with standard screws. The incidence of lag screw cutout in the standard screw group was similar to results seen in a previous study,22 but in the HA-coated screw group, no lag screw cutout occurred. This can be attributed to the osteointegrative ability of the HA-coated screws, because there were no differences between the two groups in patient age, fracture type, bone density, angle of the plate, quality of reduction, lag screw position in the femoral head, and tip apex distance.
Our results confirm previous animal and clinical studies showing that HA is very well osteointegrated, even when implanted in osteoporotic bone.23–26 A comparative animal study by Soballe et al32 showed superior osteointegration in HA-coated versus Ti components implanted in dogs with osteopenia. In a clinical study of osteoporotic wrist fractures treated with external fixation, we showed that HA-coated pins were better fixed than similar standard pins.24
In trochanteric fractures fixed with standard implants, a decrease in the femoral neck shaft angle often will occur.21 In our study, the superior fixation provided by the HA-coated screws was confirmed by the femoral neck-shaft angle analysis. With the standard screws, the femoral neck-shaft angle at 6 months was lower than it was postoperatively, whereas in the HA-coated screw group, no differences were observed. The absence of postoperative variation in the HA-coated screw group is important, because restoration of prefracture hip anatomy is an important parameter influencing functional outcome.6,15,16
Fracture impaction was lower in patients in Group B, but statistical significance was not reached. Because fracture impaction is related more closely to the sliding mechanism of the device rather than to screw purchase, the enhanced fixation of the HA-coated screws probably did not influence this outcome.
We wanted to maximize fixation in the osteoporotic bone, therefore, lag and cortical HA-coated screws were used. In the uncoated screw group, none of standard cortical screws cut out. This result indicates that HA-coated screws probably are not necessary for DHS fixation in cortical bone.
As can be expected with this patient population, average functional results (measured by the Harris hip score) were low in both groups, although in the HA-coated group, functional results were higher than in the standard screw group. We contend that the better bone purchase of the HA-coated screws (which in turn, helped to maintain the reduction with time) contributed to the superior functional outcomes. Despite the better clinical results of patients in Group B, there were no differences between the groups in Short Form-36 score. The similarities in health and mental status of this particular patient population probably affected this result.
Hydroxyapatite-coated screws have several advantages over augmentation techniques with PMMA or calcium phosphate cements. Implantation is fast and easy, the cost is lower, and special instrumentation or changes in insertion technique are not required. In addition, the safety and effectiveness of HA-coated screws has been validated in several clinical studies of external fixation.24–27
Our study confirms that correct lag screw positioning is mandatory with standard screws. However, even HA-coated screws implanted in at-risk positions did not fail. We recommend AO/ASIF lag screws coated with HA for DHS fixation of osteoporotic trochanteric fractures. These screws improved fixation, lowered the postoperative complication rate, and improved clinical outcomes. AO/ASIF screws coated with HA could be the key to optimal fracture fixation of osteoporotic and mechanically weak bone.
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