Biomechanics of pin and screw fixation of femoral neck fractures

Comparative Efficacy and Safety of Robot-Assisted vs. Freehand Screw Placement in Femoral Neck Fractures: An Updated Systematic Review and Meta-Analysis

Background: Femoral neck fractures pose significant surgical challenges with high morbidity and mortality. Traditional freehand screw placement often yields variable outcomes. Recent robotic advancements offer a promising alternative with enhanced precision. Methods: This systematic review compares the efficacy and safety of robot-assisted versus freehand techniques. A comprehensive literature search across multiple databases up to July 2024 included studies comparing both techniques. Primary outcomes were the union rate and time, functional outcomes, operative time, intraoperative parameters, and complication rates. Meta-regression analyses identified treatment response determinants. Results: Twenty-four studies (1437 patients) were included. Robot-assisted screw placement significantly improved the union rate, reduced the union time, and showed superior functional outcomes. Additionally, it resulted in shorter operative times, less intraoperative blood loss, and fewer instances of fluoroscopy and guide pin insertion. The risk of femoral neck necrosis was notably lower with robotic assistance. Meta-regression highlighted the robot type, patient age, and sample size as significant factors. Conclusions: Despite the promise of robot-assisted screw placement, limitations exist. The evidence being mainly from China raises concerns about generalizability. The lack of long-term follow-up data hinders assessment of technique durability. Unreported surgeon expertise levels and learning curves affect result validity. High initial costs and steep learning curves of robotic systems also present barriers to widespread adoption.

Mechanical Properties of Synthetic Bones Made by Synbone: A Review

Biomechanical engineers and physicists commonly employ biological bone for biomechanics studies, since they are good representations of living bone. Yet, there are challenges to using biological bone, such as cost, degradation, disease, ethics, shipping, sourcing, storage, variability, etc. Therefore, the Synbone® company has developed a series of synthetic bones that have been used by biomechanical investigators to offset some drawbacks of biological bone. There have been a number of published biomechanical reports using these bone surrogates for dental, injury, orthopedic, and other applications. But, there is no prior review paper that has summarized the mechanical properties of these synthetic bones in order to understand their general performance or how well they represent biological bone. Thus, the goal of this article was to survey the English-language literature on the mechanical properties of these synthetic bones. Studies were included if they quantitatively (a) characterized previously unknown values for synthetic bone, (b) validated synthetic versus biological bone, and/or (c) optimized synthetic bone performance by varying geometric or material parameters. This review of data, pros, cons, and future work will hopefully assist biomechanical engineers and physicists that use these synthetic bones as they develop experimental testing regimes and computational models.

Hybrid screw fixation for femoral neck fractures: Does it prevent mechanical failure?

INTRODUCTION

Traditionally, femoral neck fracture fixation has been performed using three partially threaded cancellous screws. However, fracture collapse with femoral neck shortening, and varus deformation frequently occurs due to posterior medial comminution and lack of calcar support. We hypothesize replacing the inferior neck/calcar screw with a fully threaded, length stable, screw will provide improved biomechanical stability, decrease femoral neck shortening and varus collapse.

METHODS

Ten matched cadaveric pairs (20 femurs) were randomly assigned to two screw fixation groups. Group 1 (Hybrid) utilized one fully threaded calcar screw & two partially threaded superior screws. Group 2 (PT) utilized all partially threaded screws. Specimens underwent standardized femoral neck osteotomies, 45° from the horizontal, with 5 mm posteromedial wedge removed to simulate posteromedial comminution. Screws were placed using fluoroscopic guidance. Specimens were biomechanically tested using two loading sequences: 1) Axial load applied up to 700 N, followed by cyclic loading at 2 Hz with loads of 700 to 1,400 N for 10,000 cycles. 2) All surviving constructs were cyclically loaded to failure in stepwise incremental manner with max load of 4,000 N. Paired t-tests used to compare stiffness, cycles to failure, and max load to failure (defined as 15 mm load actuator displacement).

RESULTS

Construct stiffness was 2848 ± 344 N/mm in PT vs. 2767 ± 665 for Hybrid (P = 0.628). Load to failure demonstrated, hybrid superiority with max cycles to failure (3797 ± 400 cycles) vs. (2981 ± 856 cycles in PT) (p = 0.010), and max load prior to failure (3290 ± 196 N) vs. (2891 ± 421 N in PT) (p = 0.010). No significant difference in bone mineral density was noted in any of the specimens.

CONCLUSIONS

Our study is the first to assess the biomechanical effects of hybrid fixation for femoral neck fractures. Hybrid screw configuration resulted in significantly stronger constructs, with higher axial load and increased cycles prior to failure. The advantageous mechanical properties demonstrated using a fully threaded inferior calcar screw provides a length stable construct which may prevent the common complication of excessive femoral neck shortening, varus collapse and poor functional outcome.

Does Screw Location Affect the Risk of Subtrochanteric Femur Fracture After Femoral Neck Fixation? A Biomechanical Study

BACKGROUND

Case reports suggest that there is an increased risk of subtrochanteric femur fracture after femoral neck fixation with cannulated screws when the distal-most screw is placed distal to the lesser trochanter. However, to our knowledge, there are no biomechanical data supporting this observation.

QUESTIONS/PURPOSES

(1) Is there an increased risk of subtrochanteric femur fracture after femoral neck fixation with cannulated screws in normal density and osteoporotic Sawbones when the distal-most screw is started distal to the lesser trochanter? (2) Does the screw starting point position after femoral neck fixation with cannulated screws affect load to failure when normal density and osteoporotic Sawbones are loaded through their mechanical axis?

METHODS

Normal density and osteoporotic Sawbones femora were instrumented with three cannulated screws in a triangular apex distal configuration with the distal-most screw starting either proximal to, at, or distal to the level of the lesser trochanter. Specimens were loaded along the mechanical axis to failure. The fracture location and ultimate load to failure were compared between groups.

RESULTS

The screw start point distal to the lesser trochanter resulted in a greater proportion of subtrochanteric femur fractures compared with screw start points at or proximal to the lesser trochanter in the subset of osteoporotic specimens (three of 10 specimens versus 0 of 20 specimens; p = 0.030). No subtrochanteric femur fractures were observed in the normal density specimens. Load to failure was lower when the distal-most screw was started distal to the lesser trochanter than when it was started at or proximal to the lesser trochanter (normal density subset 13,502 ± 1980 N versus 14,675 ±1528 N; osteoporotic subset 8946 ± 1509 N versus 10,026 ± 1256 N; linear regression coefficient 1127 N [95% CI 298 to 1956 N]; adjusted r = 0.71; p = 0.009).

CONCLUSIONS

A screw start point distal to the lesser trochanter was associated with subtrochanteric femur fractures in the osteoporotic subset. Additionally, there was decreased load to failure when the distal-most screw was started distal to the lesser trochanter.

CLINICAL RELEVANCE

These data suggest that avoiding a screw start point distal to the level of the lesser trochanter in femoral neck fracture fixation may decrease the risk of catastrophic subtrochanteric femur fractures, especially in patients with osteoporosis. However, it should be noted that a more overall varus screw alignment could theoretically compromise the ability to achieve compression across the fracture, with attendant implications with regard to fracture union in the acute setting.

Four Screws Diamond Configuration Fixation for Displaced, Comminuted Intracapsular Fracture Neck Femur in Young Adults

BACKGROUND

Four cannulated cancellous screws (CCS) in diamond configuration have been recommended by some authors for fixation of intracapsular neck of femur (ICNF) fracture with posterior comminution in adults. This is also supported by biomechanical studies. However, the clinical usefulness of this biomechanical advantage is not known. This study evaluates the outcomes of displaced, comminuted ICNF fracture in young adults managed by four CCS fixation in diamond configuration.

MATERIALS AND METHODS

25 patients who met the inclusion criteria were operated during the study period. Four patients were lost to followup in the first 6 weeks postoperatively, and one patient died in first 6 months of followup unrelated to surgery. Thus, twenty patients were followed up for a mean period of 33.3 months (range 25-38 months). In all patients, fracture fixation was done with four CCS in diamond configuration. Union at fracture site and avascular necrosis (AVN) of femoral head was assessed on serial plain radiographs. Functional outcome was evaluated by Harris hip score.

RESULTS

Eighteen patients had union, two patients had nonunion, and two patients had AVN of femoral head. All the sixteen patients who had union without AVN had good or excellent functional outcome.

CONCLUSIONS

The present study concludes that four CCS fixation in diamond configuration appears to be a reasonable choice of fixation for displaced fracture ICNF with comminution in young adults. However, further evaluation with better study design and larger patient population is required for definite conclusions.

Stability of L-shaped and inverted triangle fixation assemblies in treating Pauwels type II femoral neck fracture: a comparative mechanical study

PURPOSE

The aim of our study is to compare the mechanical resistance of two screw configurations in fixating type II Pauwels femoral neck fractures.

METHODS

Fifteen synthetic models of femur bones in young adults were divided into three equal groups: intact (G1), models with fixation of a 5.0-mm failure zone created in the posterior cortex of the femoral neck using an L-shaped screw arrangement (G2, n = 5), and models with an identical failure zone fixated using an inverted triangle assembly (G3, n = 5). Model strength (axial loading) and rotational deviation of the fragments were load-tested until a 5.0-mm displacement was reached (step 1) and then until failure, here considered as 10.0 mm displacement in G2 and G3 or femoral neck fracture in G1 (step 2).

RESULTS

In step 1, the mean resistance in G1 was 1593 N (standard deviation [SD] of 62 N); this value in G2 was 1261 N (SD 49 N) and in G3 was 1074 N (SD 153 N). During step 2, the value for G1 was 2247 N (SD 84 N), for G2 was 1895 N (SD 69 N), and for G3 was 1523 N (SD 280 N). G3 (the inverted triangle assembly) showed a significantly lower maximum load than the group using the L-shaped assembly (G2) and the control group (G1), which was significant using Kruskal-Wallis analysis of variance (p = 0.002).

CONCLUSION

Under test conditions in synthetic bone, fixation using a L-shaped screw assembly provides greater mechanical resistance than an inverted triangle assembly.

Cement augmentation of implants--no general cure in osteoporotic fracture treatment. A biomechanical study on non-displaced femoral neck fractures

Femoral neck fractures in the elderly are a common problem in orthopedics. Augmentation of screw fixation with bone cement can provide better stability of implants and lower the risk of secondary displacement. This study aimed to investigate whether cement augmentation of three cannulated screws in non-displaced femoral neck fractures could increase implant fixation. A femoral neck fracture was simulated in six paired human cadaveric femora and stabilized with three 7.3 mm cannulated screws. Pairs were divided into two groups: conventional instrumentation versus additional cement augmentation of screw tips with 2 ml TraumacemV+ each. Biomechanical testing was performed by applying cyclic axial load until failure. Failure cycles, axial head displacement, screw angle changes, telescoping and screw cut-out were evaluated. Failure (15 mm actuator displacement) occurred in the augmented group at 12,500 cycles (± 2,480) compared to 15,625 cycles (± 4,215) in the non-augmented group (p = 0.041). When comparing 3 mm vertical displacement of the head no significant difference (p = 0.72) was detected between the survival curves of the two groups. At 8,500 load-cycles (early onset failure) the augmented group demonstrated a change in screw angle of 2.85° (± 0.84) compared to 1.15° (± 0.93) in the non-augmented group (p = 0.013). The results showed no biomechanical advantage with respect to secondary displacement following augmentation of three cannulated screws in a non-displaced femoral neck fracture. Consequently, the indication for cement augmentation to enhance implant anchorage in osteoporotic bone has to be considered carefully taking into account fracture type, implant selection and biomechanical surrounding.

Biomechanical rationale for implant choices in femoral neck fracture fixation in the non-elderly

Femoral neck fractures represent a relatively uncommon injury in the non-elderly population often resulting from high-energy trauma. The cornerstone of their management is anatomic reduction and stable internal fixation of the femoral neck in an attempt to salvage the femoral head. Complications including avascular necrosis of the femoral head, non-union and post-traumatic osteoarthritis are not uncommon. The clinical outcomes of these patients can be improved with good pre-operative planning, optimization of surgical procedures and introduction of new improved implants and techniques. In the herein study, we attempt to describe the biomechanical properties of the hip and compare the performance of the most commonly used devices. Experimental evidence suggests that in Pauwels type III fracture patterns a cephalomedullary nail was significantly stronger in axial loading. Moreover, in unstable basicervical patterns cannulated screws (triangular configuration) demonstrated a lower ultimate load to failure, whereas in subcapital or transervical patterns both the cannulated screws (triangular configuration) and the sliding hip screw demonstrated no compromise in fixation strength. The fracture pattern appears to be the major determinant of the ideal type of implant to be selected. For a successful outcome each patient needs to be considered on an individual basis taking into account all patient and implant related factors.

Application of a medial buttress plate may prevent many treatment failures seen after fixation of vertical femoral neck fractures in young adults

Femoral neck fractures in young adults with normal bone are mostly vertically oriented and may have variable amounts of comminution, which result from shearing forces during high-energy trauma. These factors play a role in the high rate of complications after this injury, including nonunion, malunion, failure of fixation, and avascular necrosis. These problems often occur together and inter-relate, for example, nonunion or malunion frequently result from fixation failure and varus collapse of the femoral head after reconstruction. The orthopaedic surgeon's goals of obtaining and maintaining anatomic reduction until bony union have been addressed by a number of surgical approaches and fixation constructs, however, complications are still common and no consensus exists on how these problematic fractures may be best treated. For optimal treatment of vertical femoral neck fractures, anatomic reduction must be achieved and fixation must be able to resist the high shear forces across the fracture with hip motion, weight-bearing, and muscle tone. Buttress plate fixation is a common method for stabilizing fractures that require resistance to shear forces and stands as one of the basic principles of fracture care. This technique has not been widely applied to this injury pattern. We propose that the concepts of modern fracture care should be applied together for vertical femoral neck fractures in young adults. Specifically, we propose that anatomic reduction and fixation of vertically oriented femoral neck fractures with the addition of a medial buttress plate to resist shearing forces will improve on the historically high rate of complications after these difficult injuries.

Four quadrant parallel peripheral screw fixation for displaced femoral neck fractures in elderly patients

BACKGROUND

The treatment options for displaced femoral neck fracture in elderly are screw fixation, hemiarthroplasty and total hip arthroplasty based primarily on age of the patient. The issues in screw fixation are ideal patient selection, optimal number of screws, optimal screw configuration and positioning inside the head and neck of femur. The problems of screw fixation may be loss of fixation, joint penetration, avascular necrosis of femoral head, nonunion, prolonged rehabilitation period and the need for second surgery in failed cases. We hereby present results of a modified screw fixation technique in femoral neck fractures in patients ≥50 years of age.

MATERIALS AND METHODS

Patients ≥50 years of age (range 50-73 years) who sustained displaced femoral neck fracture and fulfilled the inclusion criteria were enrolled in this prospective study. They were treated with closed reduction under image intensifier control and cannulated cancellous screw fixation. Accurate anatomical reduction was not aimed and a cross sectional contact area of >75% without varus was accepted as good reduction. Four screws were positioned in four quadrants of femoral head and neck, as parallel and as peripheral as possible. Radiological and functional results were evaluated periodically. Sixty four patients who could complete a minimum followup of two years were analyzed.

RESULTS

Radiologically, all fractures healed after mean duration of 10 weeks (range 8-12 weeks). There was no avascular necrosis. Nonanatomical healing was observed in 45 cases (70%). All patients except one had excellent functional outcome and could do cross-legged sitting and squatting. Chondrolysis with progressive head resorption was seen in one case, which was converted to total hip arthroplasty.

CONCLUSION

Closed reduction and cannulated cancellous screw fixation gives satisfactory functional results in large group of elderly patients. The four quadrant parallel peripheral (FQPP) screw fixation technique gives good stability, allows controlled collapse, avoids fixation failure and achieves predictable bone healing in displaced femoral neck fracture in patients ≥50 years of age.

Biomechanical Measurements of Surgical Drilling Force and Torque in Human Versus Artificial Femurs

Few experimental studies have examined surgical drilling in human bone, and no studies have inquired into this aspect for a popular commercially-available artificial bone used in biomechanical studies. Sixteen fresh-frozen human femurs and five artificial femurs were obtained. Cortical specimens were mounted into a clamping system equipped with a thrust force and torque transducer. Using a CNC machine, unicortical holes were drilled in each specimen at 1000 rpm, 1250 rpm, and 1500 rpm with a 3.2 mm diameter surgical drill bit. Feed rate was 120 mm/min. Statistical significance was set at p < 0.05. Force at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (198.4 ± 14.2 N, 180.6 ± 14.0 N, and 176.3 ± 11.2 N) and artificial femurs (87.2 ± 19.3 N, 82.2 ± 11.2 N, and 75.7 ± 8.8 N). For human femurs, force at 1000 rpm was greater than at other speeds (p ≤ 0.018). For artificial femurs, there was no speed effect on force (p ≥ 0.991). Torque at increasing spindle speed (1000 rpm, 1250 rpm, and 1500 rpm), respectively, showed a range for human femurs (186.3 ± 16.9 N·mm, 157.8 ± 16.1 N·mm, and 140.2 ± 16.4 N·mm) and artificial femurs (67.2 ± 8.4 N·mm, 61.0 ± 2.9 N·mm, and 53.3 ± 2.9 N·mm). For human femurs, torque at 1000 rpm was greater than at other speeds (p < 0.001). For artificial femurs, there was no difference in torque for 1000 rpm versus higher speeds (p ≥ 0.228), and there was only a borderline difference between the higher speeds (p = 0.046). Concerning human versus artificial femurs, their behavior was different at every speed (force, p ≤ 0.001; torque, p < 0.001). For human specimens at 1500 rpm, force and torque were linearly correlated with standardized bone mineral density (sBMD) and the T-score used to clinically categorize bone quality (R ≥ 0.56), but there was poor correlation with age at all speeds (R ≤ 0.37). These artificial bones fail to replicate force and torque in human cortical bone during surgical drilling. To date, this is the largest series of human long bones biomechanically tested for surgical drilling.

Contemporary management of femoral neck fractures: the young and the old

The optimal management strategy for femoral neck fractures remains highly debated. The femoral neck is intracapsular and the vascular supply is fragile. Furthermore, the curvature of the proximal femur results in high mechanical stresses through the femoral neck. Poor outcomes of nonunion and avascular necrosis (AVN) are common. This chapter reviews the current evidence with respect to the treatment principles of femoral neck fractures in two distinct patient populations: "young" and "old." Contemporary controversies including surgical timing, choice of implant, arthroplasty options, nonoperative management, capsulotomy, and associated complications will be discussed.

[Percutaneous screw osteosynthesis of femoral neck fractures in the elderly. Subtrochanteric fractures as severe complications]

Percutaneous osteosynthesis by cannulated screws is a well established method for the treatment of non-displaced femoral neck fractures, especially in elderly patients with comorbidities due to low operative invasiveness. Optimal screw placement is still under discussion. There are only few literature references concerning the complication of subtrochanteric femoral fractures. We performed a review of the literature and a retrospective analysis of our patients treated by this form of osteosythesis from 01.04.2004 to 30.09.2009 searching for screw placement and the incidence of subtrochanteric femoral fractures. This complication was found in 2 of our 35 patients (5.7%) without adequate trauma, such as a stumble or fall having occurred.

Management of femoral neck fractures in young adults

Femoral neck fractures in young adults are uncommon and often the result of high-energy trauma. They are associated with higher incidences of femoral head osteonecrosis and nonunion. Multiple factors can play a significant role in preventing these devastating complications and contribute to a good outcome. While achieving an anatomic reduction and stable internal fixation are imperative, other treatment variables, such as time to surgery, the role of capsulotomy and the fixation methods remain debatable. Open reduction and internal fixation through a Watson-Jones exposure is the recommended approach. Definitive fixation can be accomplished with three cannulated or noncannulated cancellous screws. Capsulotomy in femoral neck fractures remains a controversial issue and the practice varies by trauma program, region and country. Until there is conclusive data (i.e. prospective and controlled) we recommend performing a capsulotomy. The data available is inconclusive on whether this fracture should be operated emergently, urgently or can wait until the next day. Until there is conclusive data available, we recommend that surgery should be done on an urgent basis. The key factors in treating femoral neck fractures should include early diagnosis, early surgery, anatomic reduction, capsular decompression and stable internal fixation.

The Value of Washers in Cannulated Screw Fixation of Femoral Neck Fractures

BACKGROUND

Given the limited evidence to support the technical aspects of screw placement for treatment of femoral neck fractures, we conducted an observational study to evaluate demographic and radiographic variables associated with fixation failure.

METHODS

Eligible patients with femoral neck fractures were treated with multiple cannulated screws across three academic centers during a 6-year period. The following variables were evaluated for their predictive value for fixation failure: age, gender, fracture type, presence of comminution, total number of screws, the absence of a washer, the screw configuration, reduction quality, the distance of the most inferior screw to the inferior neck, and screw alignment. Variables were evaluated separately and in a multivariable regression model.

RESULTS

Eighty patients were included in the study. The overall failure rate was 30%. We identified four variables associated with fixation failure. These included the lack of washers (odds ratio [OR], 11.2; p = 0.03), imperfect quality of reduction (OR, 9.7; p < 0.01), age greater than 75 years (OR, 5.1; p = 0.04), and displaced versus undisplaced fracture type (OR, 3.8; p < 0.01). These four variables accounted for 43% of the variability in fixation failure (R(2) = 0.43). All other variables including the distance of the most inferior screw to the inferior/medial neck were found to be not significant.

CONCLUSION

This study confirms previous findings in the literature that increased age, a displaced fracture type, and poor reduction increase the risk of fixation failure. Contradictory to current belief, there was no significant association between the distance of the inferior screw to the inferior/medial femoral neck cortex and fixation failure. A novel finding of the present study is that the use of washers significantly decreases the risk of fixation failure.

Subtrochanteric fracture after cannulated screw fixation of femoral neck fractures: a report of four cases

Subtrochanteric fractures after screw or pin fixation of femoral neck fractures are a recognized complication. No literature is available on this complication after fixation using the recently popularized cannulated screws. We present our experience in treating four of these complications. The common denominator for all four patients seemed to be an entry point in the lateral cortex below the level of the most inferior edge of the lesser trochanter.

Vertical shear fractures of the femoral neck. A biomechanical study

A biomechanical cadaver study was performed to compare the strength and stability of three cannulated cancellous lag screws with a sliding hip screw for fixation of a vertically oriented fracture of the femoral neck (Pauwels Type III). Using eight matched pairs of human cadaveric femurs, vertically oriented femoral neck osteotomies were created, reduced, and randomized to one of the two fixation methods. The constructs were tested with incremental axial loading from 100 N to 1200 N and cyclical loading at 1000 N for 10,000 cycles; fracture displacements and ultimate load to failure were determined. The specimens stabilized using a sliding hip screw showed less inferior femoral head displacement, less shearing displacement at the osteotomy site, and a much greater load to failure than did those stabilized with multiple cancellous lag screws. These results support the use of a sliding hip screw for treatment of vertically oriented fractures of the femoral neck.

Internal fixation of femoral neck fractures with posterior comminution: a biomechanical study

OBJECTIVES

This study was performed to determine whether four cancellous lag screws provide significantly improved rigidity and fixation strength compared with three screws for fixation of displaced femoral neck fractures with posterior comminution.

DESIGN

Biomechanical cadaver study.

INTERVENTION

Eight pairs of mildly osteopenic femurs were selected, and each pair was fixed with three or four cancellous lag screws (randomly assigned) after the creation of a simulated femoral neck fracture with posterior comminution. A separate comparison with an unmatched group of six similar femurs with a simulated femoral neck fracture without posterior comminution and instrumented with three screws was performed to investigate the effect of posterior comminution.

MAIN OUTCOME MEASUREMENT

The specimens were non-destructively tested to determine fixation rigidity in axial and anterior loading. Cyclic axial loading was then performed for 10,000 cycles; the femurs were retested for rigidity and finally were axially loaded until failure.

RESULTS

The femurs with a posterior defect stabilized with three screws had significantly less resistance to axial and anterior displacement and sustained significantly lower axial loads to failure than those stabilized with four screws. The specimens instrumented with three screws without a posterior defect exhibited greater resistance to displacement in anterior loading and sustained greater axial loads to failure than those with a posterior defect stabilized with three screws.

CONCLUSION

This study suggests that there are benefits to using four screws for fixation of femoral neck fractures with posterior comminution.

In vitro femoral stiffness after femoral neck osteotomy and osteosynthesis with defined surgical errors

In our search for an osteosynthesis device that would tolerate the surgical errors of the inexperienced surgeon, we tested in vitro femoral stiffness in 75 human osteoporotic femora after internal fixation of a cervical neck osteotomy using three commonly used devices: two von Bahr screws (A. Ericsson AB, Sweden), two cannulated screws (Uppsala type, Olmed AB, Sweden), and two hookpins (LiH, PSAB, Sweden). The first device has its main grip in the cancellous bone by threads; the second has grip in cancellous and subchondral bone by threads; and the third, which has no threads, has its grip in cancellous bone by a hook pin. The intact specimen was in all instances stiffer (22-63%) than the osteosynthesized specimen (p < 0.001). An osteosynthesized femur with perfectly reduced bone ends was 14-23% stiffer than when reduction of the bone ends was insufficient, irrespective of device malposition (p < 0.001). Insufficient reduction of the osteotomy leaving a 20 degrees dorsal angulation of the femoral head combined with too far ventrally placed screws resulted in the lowest femoral stiffness. If reduction of osteotomy was sufficient, screws placed too far ventrally or converging screws did not result in decreased stiffness compared with optimal screw placement. Irrespective of the quality of reduction, osteosynthesis with the Uppsala screw resulted in all instances in a higher stiffness than using the other devices (p < 0.01). With the Uppsala screw design, femoral stiffness after optimal osteosynthesis was reduced by 22% compared with the intact femur, and in the most unfavorable position with combined malreduction and malpositioning it was reduced by 42%. Corresponding values for the von Bahr screws were 29% and 46%, respectively, and for the LiH screws 47% and 63%, respectively. Use of a device with threads and grip in the subchondral bone is recommended for fixation of femoral neck fractures in osteoporotic bone. Furthermore, the importance of anatomical reduction for fracture fixation is emphasized.

Orthopedic implants: a guide to radiographic analysis

Orthopedic technology is growing at a rapid rate. New implants reflect this technology whether it pertains to fracture fixation, joint replacement, spine stabilization, or soft-tissue reconstruction. This monograph is designed to assist both the radiologist and the orthopedist in accurately assessing plain radiographs of commonly used orthopedic implants. We discuss the biologic response of the surrounding bone to these devices; herein lies the key to understanding the radiographic presentation of these devices.