Aims

This study investigates head-neck taper corrosion with varying head size in a novel hip simulator instrumented to measure corrosion related electrical activity under torsional loads.

Aims

Using tibial shaft fracture participants from a large, multicentre randomized controlled trial, we investigated if patient and surgical factors were associated with health-related quality of life (HRQoL) at one year post-surgery.

Purpose

The Birmingham Mid-Head Resection (BMHR) is a bone-conserving, short-stem alternative to hip resurfacing for patients with compromised femoral head anatomy. It is unclear, however, if an uncemented, metaphyseal fixed stem confers a mechanical advantage to that of a traditional hip resurfacing in which the femoral prosthesis is cemented to the prepared femoral head. Thus, we aimed to determine if a metaphyseal fixed, bone preserving femoral component provided superior mechanical strength in resisting neck fracture compared to a conventional hip resurfacing arthroplasty.

Purpose: Optimal fixation for comminuted proximal humerus fractures is controversial. Complications using locked plates have been addressed by anatomic reduction or medial cortical support. The current study measured relative mechanical contributions of varus malalignment and medial cortical support.

Method: Forty synthetic humeri were divided into three groups, osteotomized, and fixed at 0, 10, and 20 degrees of varus malreduction with locked proximal humerus plates (AxSOS, Global model, Stryker, Mahwah, NJ, USA). This simulated mechanical medial support with the cortex intact. Axial, torsional, and shear stiffness were experimentally measured. Half of the specimens in each of the groups underwent a second osteotomy to create a segmental defect which simulated loss of medial support with the cortex removed. Axial, torsional, and shear stiffness experiments were repeated, followed by shear load to failure in 20 degrees of abduction.

Results: For isolated malreduction with the cortex intact, the repair construct at 0 degrees showed statistically equivalent or higher axial, torsional, and shear stiffness than other groups assessed. Subsequent removal of cortical support in half the specimens resulted in a drastic effect on axial, torsional, and shear stiffness at all varus angles. Repair constructs with the cortex intact at 0 and 10 degrees resulted in mean shear failure forces of 12965.4 N and 9341.1 N, respectively. These were statistically higher (p< 0.05) compared to most other groups tested. Specimens failed mainly by plate bending as the femoral head was pushed down medially and distally.

Conclusion: Anatomic reduction with the medial cortex intact was the stiffest construct after a simulated two-part fracture. This study also supports the practice of achieving medial cortical support by fixing proximal humeral fractures in varus if necessary. This may be preferable to fixing the fracture in anatomic alignment when there is a medial fracture gap.

Purpose: Minimizing tip-apex distance (TAD) has been shown to reduce clinical failure of extramedullary sliding hip screws used to fix peritrochanteric fractures. There is debate regarding the optimal position of the lag screw in the femoral head when a cephalomedullary nail is used to treat a peritrochanteric fracture. Some authors suggest the TAD should be minimized as with an extramedullary sliding hip screw, while others suggest the lag screw should be placed inferior within the femoral head. The primary goal of this study was to determine which of 5 possible lag screw positions in the femoral head provides greatest mechanical stiffness and/or load-to-failure for an unstable peritrochanteric fracture treated with a cepha-clomedullary nail. The secondary goal was to determine if there is a linear correlation between implant-femur mechanical stiffness and/or load to failure (dependent variables) with a series of five radiographic measurements (independent variables) of distance from the lag screw tip to the femoral head apex.

Method: Long Gamma 3 Nails (Stryker, Mahwah, NJ) were inserted into 30 left synthetic femurs (Pacific Research Laboratories, Vashon, WA). An unstable four-part fracture was created, anatomically reduced, and repaired using one of 5 lag screw placements in the femoral head:

superior (n=6),

All specimens were radiographed in the anterioposterior and lateral planes, and radiographic measurements including TAD and a calcar referenced tip-apex distance (CalTAD) were calculated. All specimens were tested for axial, lateral, and torsional stiffness, and then loaded-to-failure in the axial position using an Instron 8874 (Canton, MA). ANOVA was used to compare means of the five treatment groups. Linear regression analysis was used to compare stiffness and load-to-failure (dependant variables) with radiographic measurements (independent variables). A post hoc power analysis was performed.

Results: The inferior lag screw position had significantly greater mean axial stiffness than superior (p< 0.01), anterior (p=0.02) and posterior (p=0.04) positions. Analysis revealed significantly less mean torsional stiffness for the superior lag screw position compared to other lag screw positions (p< 0.01 all 4 pairings). No statistical differences were noted for lateral stiffness. Superior and central lag screw positions had significantly greater mean load-to-failure than anterior (p< 0.01 and p=0.02) and posterior (p< 0.01 and p=0.05) positions.

There were significant negative linear correlations between stiffness tests with CalTAD, and load-to-failure with TAD. Power was greater than 95% for axial stiffness, torsional stiffness and load-to-failure tests.

Conclusion: Position of the lag screw in the femoral head affects the biomechanical properties of the implant-femur construct. Central placement of the lag screw with minimization of TAD may provide the best combination of stiffness and load-to-failure.

Purpose: Cephalomedullary nails rely on a large lag screw that provides fixation into the femoral head. There is an option to statically lock the lag screw (static mode) or to allow the lag screw to move within the nail to compress the intertrochanteric fracture (dynamic mode). The purpose of this study was to compare the biomechanical stiffness of static and dynamic modes for a cephalomedullary nail used to fix an unstable peritrochanteric fracture.

Method: Thirty intact synthetic femur specimens (Model #3406, Pacific Research Laboratories, Vashon, WA) were potted into cement blocks distally for testing on an Instron 8874 (Instron, Canton, MA). A long cephalomedullary nail (Long Gamma 3 Nail, Stryker, Mahwah, NJ) was then inserted into each of the femurs. An unstable four-part fracture was created, anatomically reduced, and the cephallomedullary nail was reinserted. Mechanical tests were conducted for axial, lateral, and torsional stiffness with the lag screws in:

static and

A paired student’s t test was used to compare the 2 modes.

Results: The axial stiffness of the cephalomedullary nail was significantly greater (p< 0.01) in the static mode (484.3±80.2N/mm) than in the dynamic mode (424.1±78.0N/mm) (Fig.2A). Similarly, the lateral bending stiffness of the nail was significantly greater (p< 0.01) in the static mode (113.9±8.4N/mm) than in the dynamic mode (109.5±8.8N/mm). The torsional stiffness of the nail was significantly greater (p=0.02) in the dynamic mode (114.5±28.2N/mm) than in the static mode (111.7±27.0N/mm).

A post hoc power analysis with & #945;=0.05 and & #946;=0.20 revealed that the paired t test on 30 samples was sufficiently powered to determine a difference in mean axial stiffness of 33.0N/mm (6.8% of static stiffness), a difference in mean lateral bending stiffness of 3.6N/mm (3.2% of static stiffness) and a difference in mean torsional stiffness of 3.4N/mm (3.0% of static stiffness).

Conclusion: Our results show that there is a 60N/mm reduction in axial stiffness of the cephalomedullary nail when the lag screw is changed from static to dynamic mode. This represents a 12.4% reduction in axial stiffness with a change from axial to dynamic modes which may be clinically significant. The differences in lateral (4.4N/mm, 3.9%) and torsional (2.8N/mm, 2.4%) are small enough that they are likely not clinically significant. We felt that a difference of greater than 10% in axial stiffness and a difference of greater than 5% in lateral or torsional stiffness would be clinically significant. Our study was adequately powered to detect these differences. Given the significant reduction in axial stiffness with dynamization of the cephalomedullary nail construct, we recommend use of the static mode when treating unstable peritrochanteric fractures with a cephalomedullary nail.

Minimizing tip-apex distance has been shown to reduce clinical failure of sliding hip screws used to fix peritro-chanteric fractures. The purpose of this study was to determine if such a relationship exists for the position of the lag screw in the femoral head using a cephalomedullary device.

Methods: Thirty intact synthetic femur specimens (Model #3406, Pacific Research Laboratories, Vashon, WA) were potted into cement blocks distally for testing on an Instron 8874 (Instron, Canton, MA). A long cephalomedullary nail (Long Gamma 3 Nail, Stryker, Mahwah, NJ) was inserted into each of the femurs. An unstable four-part fracture was created, anatomically reduced, and repaired using one of 5 lag screw placements in the femoral head:

Superior (N=6),

Mechanical tests were repeated for axial, lateral and torsional stiffness. All specimens were radiographed in the anterioposterior and lateral planes and tip-apex (TAD) distance was calculated. A calcar referenced tip-apex distance (CalTAD) was also calculated.

ANOVA was used to compare means of the five treatment groups. Linear regression analysis was used to compare axial, lateral and torsional stiffness (dependant variables) to both TAD and CalTAD (independent variables).

Results: ANOVA testing proved that the mean axial (p< 0.01) and torsional stiffness (p< 0.01) between the 5 groups was significantly different, but lateral stiffness was not statistically different (p=0.494). Post hoc analysis showed that the inferior lag screw position provided significantly higher mean axial stiffness (568.14±66.9N/ mm) than superior (428.0±45.6N/mm; p< 0.01), anterior (443.2±45.4N/mm; p=0.02) and posterior (456.7±69.3N/ mm; p=0.04) lag screw positions. There was no significant difference in mean axial stiffness between inferior (568.14±66.9N/mm) and central (525.4±81.7N/mm) lag screw positions (p=0.77). Post hoc analysis revealed significantly less mean torsional stiffness for the superior lag screw position compared to other lag screw positions (p< 0.01 all 4 pairings). There were no significant correlations between TAD and axial (r=−0.33, p=0.08), lateral (r=−0.22, p=0.24) or torsional (r=0.08, p=0.69) stiffness. There were significant correlations between CalTAD and axial (r=−0.66, p< 0.01), lateral (r=−0.38, p=0.04) and torsional (r=−0.38, p=0.04) stiffness.

Discussion: Our results suggest that placement of the lag screw inferiorly in the femoral head when using a cephalomedullary nail to treat an unstable peritrochanteric fracture results in the stiffest construct in axial and torsional biomechanical testing. A simple radiographic measurement, CalTAD, provides an intraoperative method of determining optimal cephalomedullary nail lag screw position to achieve greatest construct stiffness.

Purpose: The mechanical behavior of human scapholunate ligaments is not described well in the literature regarding torsion. Presently, intact scapholunate specimens were mechanically tested in torsion to determine if any tensile forces were generated as a result.

Method: Scapholunate specimens (n=19) were harvested and inspected visually. Scaphoid and lunate bones were potted in square chambers using epoxy cement. The interposing ligaments remained exposed. Specimens were mounted in a specially designed test jig and remained at a fixed axial length during testing. Using angular displacement control, ligaments were subjected to a torsional motion regime that included cyclic preconditioning (25 cycles, 1 Hz, triangular wave, 5 deg max), ramp-up to 15 deg at 180 deg/min, stress relaxation for 120 sec duration, ramp-down to 0 angulation at 180 deg/min, rest period for 5–10 minutes, and torsion-to-failure at 180 deg/min. Torque and axial tension were monitored simultaneously.

Results: Tests showed a coupled linear relationship between applied torsion and the resultant tensile forces generated for the ligament during ramp-up (Torsion/Tension Ratio = 38.86 +/− 29.00 mm, Linearity Coefficient R-squared = 0.89 +/− 0.15, n=19), stress relaxation (Ratio = 23.43 +/− 15.84 mm, R-squared = 0.90 +/− 0.09, n=16), and failure tests (Ratio = 38.81 +/− 26.39 mm, R-squared = 0.77 +/− 0.20, n=16). No statistically significant differences were detected between the Torsion/Tension ratios (p=0.13) or between the linearity (R-squared) of the best-fit lines (p> 0.085).

Conclusion: A strong linear relationship between applied torsion and resulting tensile forces for the ligament was exhibited during all testing phases. This may suggest that there is interplay between torsion and tension in both the stabilization of the scapholunate ligament during normal physiological motion and during resistance to injury processes. This is the first report in the literature of the coupling of torsion with tension for the scapholunate ligament.

Femur fractures are a complication of hip arthroplasty. When the stem is well fixed, fracture fixation is the preferred treatment option. Numerous fixation methods have been advocated, using plates and/or allograft struts. The study was conducted to determine the biomechanical characteristics of three constructs currently used for fixation of these fractures.

Vancouver type B1 periprosthetic femur fractures were created distal to a cemented hip stem implanted in third generation composite femurs. The fractures were fixed with one of three constructs: 1- A non-locking plate and allograft strut (NLP-A) 2- A locking plate and allograft strut (LP-A) 3- A locking plate alone. (LP) The struts were held in place with cables. There were five specimens in each group. Following fixation, the constructs underwent sinusoidal cyclic loading from 200 to 1200 N for 100000 cycles. Stiffness of the constructs was determined in bending, torsion and axial compression before and after cyclic loading. Axial load to failure was also determined.

Overall, cyclic loading had little effect on the mechanical properties of these constructs. The two constructs with allografts were significantly stiffer in coronal plane bending than the construct consisting of only a locking plate. There were no significant differences in axial or torsional stiffness between the constructs. Load to failure of the NLP-A (4095 N) and LP-A (4007 N) constructs was significantly greater than the LP construct (3398 N) (p=0.023 and p=0.044 respectively).

All three constructs tested retained their mechanical characteristics following 100000 cycles of loading. Our initial concerns that the cables holding the allograft strut would loosen appear unfounded. Allograft strut-plate constructs are stiffer in bending and have a higher load to failure than a stand-alone locking plate. When an allograft plate construct is chosen, locking screws provide no mechanical advantage in this experimental model.

Gaining stable fixation in cases of recalcitrant non-unions can be challenging. These cases can be accompanied by a segmental bone defect and disuse osteopenia. One strategy to gain stable fixation is the use of allografts. Both cortical struts and intramedullary fibular allografts have been used for this purpose in the femur, tibia and humerus. The present study aims to compare the mechanical properties a locking plate, an intramedullary fibular strut allograft and a cortical strut allograft in a femur model of segmental bone defect.

A transverse mid-shaft osteotomy was performed in fifteen third generation large composite femurs. Twelve millimeters of bone was resected to create a segmental bone defect. Fixation was undertaken as follows: Construct F (Fibula): Lateral Non Locking plate and Intramedullary Fibula Allograft Construct LP (Locking Plate): Lateral Locking Plate Constrcut S (Strut): Lateral Non-Locking Plate and Medial Cortical Strut Allograft Axial, Torsional and Bending Stiffness as well as Load-to-Failure were determined using an Instron 8874 materials testing machine.

Overall, construct S was the stiffest, construct F intermediate and construct LP the least stiff. Specifically, the S construct was significantly (p< 0.05) stiffer than the two other constructs in the axial, coronal plane bending, sagital plane bending and torsional modes. Construct F was significantly stiffer than construct LP in the axial and coronal plane bending modes only. Both the S construct (6108 N) and the F construct (5344 N) had a greater Load-to-Failure than the LP construct (2855 N) (p=0.005 and 0.001 respectively).

The construct with a lateral non-locking plate and a medial allograft strut was stiffer and had a higher load-to-failure than the construct consisting of a stand-alone locking plate. An intramedullary fibular allograft with a lateral non-locking plate had intermediate characteristics. Other factors, such as anatomic and biologic considerations need to be considered before choosing one of the above constructs. The allograft procedures should only be used once soft tissue coverage has been obtained and any infection eradicated.

This study examines the biomechanical performance of five types of fixation techniques in a model of pathological fracture of the diaphyseal humerus.

In forty synthetic humeri, a hemi cylindrical defect centered in the middle third of the diaphysis was created. A transverse fracture was created through the centre of each defect. The bones were randomly assigned to five groups. Group A was fixed with standard ten hole DCP plates centered over the defect with five screws inserted on either end. In group B, the screw holes were injected with bone cement and then the screws and plate were reapplied while the cement was still soft. The defect was also filled with cement. Group C was fixed by injecting the cement into the entire intramedullary canal. The fracture was then reduced and the screws and plate were applied once the cement had hardened. In group D, the specimens were fixed with locked antegrade IM nail with one proximal and one distal interlocking screw. Group E was same as D except that the defect was filled with cement. Each specimen was tested in external rotation to failure by fracture.

There was no significant difference in torsional stiffness between groups B, C, and E (P> 0.16), whereas there were differences between all other groups using pairwise comparisons(p< 0.001). Groups B, C, and E were of highest stiffness followed by A and then D. Group C had the highest torque to failure, followed by groups A/B and then D/E. Total cumulative energy to failure for group C was statistically greater than each of B, D, and E (p< 0.005), but not different from A, though it approached significance (p=0.057).

This study demonstrates that, in a model of a fracture through a hemicylindrical defect in the middiaphysis of the humerus, fixation with a broad ten-hole dynamic compression plate after filling the entire medullary canal with cement is associated with the highest torque to failure and energy to failure with torsional forces. This fixation technique may best accomplish the clinical goal of maximal initial stability.

We aimed to establish if radiological parameters, dual energy x-ray absorbtiometry (DEXA) and quantitative CT (qCT) could predict the risk of sustaining a femoral neck fracture following hip resurfacing. 21 unilateral fresh frozen femurs were used. Each femur had a plain AP radiograph, DEXA scan and quantitative CT scan. Femurs were then prepared for a Birmingham Hip Resurfacing femoral component with the stem shaft angle equal to the native neck shaft angle. The femoral component was then cemented onto the prepared femoral head. No notching of the femoral neck occurred in any specimens. A repeat radiograph was performed to confirm the stem shaft angle. The femurs were then potted in a position of single leg stance and tested in the axial direction to failure using an Instron mechanical tester. The load to failure was then analysed with the radiological, DEXA and qCT parameters using multiple regression. The strongest correlation with the load to failure values was the total mineral content of the femoral neck at the head/neck junction using qCT r= 0.74 (p< 0.001). This improved to r=0.76 (p< 0.001) when neck width was included in the analysis. The total bone mineral density measurement from the DEXA scan showed a correlation with the load to failure of r=0.69 (p< 0.001). Radiological parameters only moderately correlated with the load to failure values; neck width (r=0.55), head diameter (r= 0.49) and femoral off-set (r=0.3). This study suggests that a patient’s risk of femoral neck fracture following hip resurfacing is most strongly correlated with total mineral content at the head/neck junction and bone mineral density. This biomechanical data suggests that the risk of post-operative femoral neck fracture may be most accurately identified with a pre-operative quantitative CT scan through the head/neck junction combined with the femoral neck width.

Introduction: We aimed to examine the effect of neck notching during hip resurfacing on the strength of the proximal femur.

Methods: Third generation composite femurs that have been shown to replicate the biomechanical properties of human bone were utilised. Imageless computer navigation was used to position the initial guide wire during head preparation. Six specimens were prepared without a superior notch being made in the neck of the femur, six were prepared in an inferiorly translated position to cause a 2mm notch in the superior femoral neck and six were prepared with a 5mm notch. All specimens had radiographs taken to ensure that the stem shaft angle was kept constant. The specimens were then loaded to failure in the axial direction with an Instron mechanical tester.

A three dimensional femoral finite element model was constructed and molded with a femoral component constructed from the dimensions of a Birmingham Hip Resurfacing. The model was created with a superior femoral neck notch of increasing depths.

Results: The 2mm notched group (mean load to failure 4034N) were significantly weaker than the un-notched group (mean load to failure 5302N) when tested to failure (p=0.017). The 5mm notched group (mean load to failure 3121N) were also significantly weaker than the un-notched group (p=0.0003) and the 2mm notched group (p=0.046). All fractures initiated at the superior aspect of the neck, at the component bone interface. The finite element model revealed increasing Von Mises stresses with increasing notch depth.

Discussion: A superior notch of 2mm in the femoral neck weakens the proximal femur by 24% and a 5mm notch weakens it by 41%. This study provides biomechanical evidence that notching of the femoral neck may lead to an increased risk of femoral neck fracture following hip resurfacing due to increasing stresses in the region of the notch.

A total of 20 pairs of fresh-frozen cadaver femurs were assigned to four alignment groups consisting of relative varus (10° and 20°) and relative valgus (10° and 20°), 75 composite femurs of two neck geometries were also used. In both the cadaver and the composite femurs, placing the component in 20° of valgus resulted in a significant increase in load to failure. Placing the component in 10° of valgus had no appreciable effect on increasing the load to failure except in the composite femurs with varus native femoral necks. Specimens in 10° of varus were significantly weaker than the neutrally-aligned specimens.

The results suggest that retention of the intact proximal femoral strength occurs at an implant angulation of ≥ 142°. However, the benefit of extreme valgus alignment may be outweighed in clinical practice by the risk of superior femoral neck notching, which was avoided in this study.

Introduction: It has been suggested that femoral component alignment in the coronal plane affects the risk of sustaining femoral neck fracture following hip resurfacing. Previous literature suggests that increasing the stem shaft angle to an extreme valgus position produces the most favourable biomechanical properties following femoral component insertion. We examined the effects of femoral component alignment during hip resurfacing on proximal femur strength.

Methods: 3^rd generation composite femurs shown to replicate biomechanical properties of human bone were used. The bones were secured in a position of single leg stance and tested with an Instron mechanical tester. Imageless computer navigation was used to position the guide wire during femoral head preparation. Specimens were placed in 115, 125 and 135 degrees of stem shaft angulation. No notching was made in the femoral neck during head preparation. The femoral components were cemented in place. Radiographs were taken ensuring that stem shaft angles were correct. Specimens were loaded to failure in the axial direction.

Results: A component position of 115 degrees compared to 125 degrees reduced load to failure from 5475N to 3198N (p=0.009). A position of 135 degrees (5713N) compared to 125 degrees (5475N) did not significantly alter the load to failure (p=0.347). Component positioning at a stem shaft angle below 125 degrees resulted in a significant reduction in strength of the proximal femur. Placement of the component at 115 degrees reduced the load to failure by 42%.

Discussion: Our findings suggest that a varus orientation may be at risk for causing femoral neck fracture. The advantages of increasing valgus angle beyond 125 degrees may not provide as much reduction in the incidence of femoral neck fracture as previously suggested, particularly when considering the inherent risk of femoral neck notching in these positions.

Introduction: Notching of the femoral neck during preparation of the femur during hip resurfacing has been associated with an increased risk of femoral neck fracture. We aimed to evaluate this with the use of a finite element model.

Methods: A three dimensional femoral model was used and molded with a femoral component constructed from the dimensions of a Birmingham Hip Resurfacing. Multiple constructs were made with the component inferiorly translated in order to cause a notch in the superior femoral neck. The component angulation was kept constant. Once constructed the model was imported into the Ansys finite element model software for analysis. Elements within the femoral model were assigned different material properties depending on cortical and cancellous bone distributions. Von Misses stresses were evaluated near the notches and compared in each of the cases.

Results: In the un-notched case the maximum Von Mises stress was only 40MPa. However, with the formation of a 1mm notch the stress rose to 144MPa and in the 4 mm notch the stress increased to 423MPa. These values demonstrated that a 1mm notch increased the maximum stress by 361% while a 4mm notch increased the maximum stress by 1061%.

Discussion: This study demonstrated that causing a notch in the superior femoral neck dramatically increases the stress within the femoral neck. This may result in the weakening of the femoral neck and potentially predispose it to subsequent femoral neck fracture. The data suggests that even a small notch of 1mm may be detrimental in weakening the femoral neck by dramatically increasing the stress in the superior neck. This study suggests that any femoral neck notching should be avoided during hip resurfacing.

Introduction: It has been suggested that notching of the femoral neck during hip resurfacing weakens the proximal femur and predisposes to femoral neck fracture. We aimed to examine the effect of neck notching during hip resurfacing on the strength of the proximal femur.

Methods: 3^rd generation composite femurs that have been shown to replicate the biomechanical properties of human bone were utilised. The bone was secured in a position of single leg stance and tested with an Instron mechanical tester. Imageless computer navigation was used to position the initial guide wire during head preparation. Six specimens were prepared without a superior notch being made in the neck of the femur, six were prepared in an inferiorly translated position to cause a 2mm notch in the superior femoral neck and six were prepared with a 5mm notch. The femoral component was then cemented in place. All specimens had radiographs taken to ensure that the stem shaft angle was kept constant. The specimens were then loaded to failure in the axial direction.

Results: The 2mm notched group (mean load to failure 4034N) were significantly weaker than the un-notched group (mean load to failure 5302N) when tested to failure (p=0.017). The 5mm notched group (mean load to failure 3121N) were also significantly weaker than the un-notched group (p=0.0003) and the 2mm notched group (p=0.046). All fractures initiated at the superior aspect of the neck, at the component bone interface. All components were positioned in the same coronal alignment +/−2 degrees.

Discussion: A superior notch of 2mm in the femoral neck weakens the proximal femur by 24% and a 5mm notch weakens it by 41%. This study provides biomechanical evidence that notching of the femoral neck may lead to an increased risk of femoral neck fracture following hip resurfacing.

Purpose: The purpose of this study was to compare the biomechanical behavior of locking plates to conventional plate and allograft constructs for the treatment of periprosthetic femoral fractures.

Methods: Twenty synthetic femora were tested in axial compression, lateral bending and torsion to characterize initial stiffness and stiffness following fixation of an osteotomy created at the tip of a cemented femoral component. Stiffness was tested with and without a 5mm gap. Axial load to failure was also tested. Four constructs were tested: Construct A – Synthes locked plate with unicortical locked screws proximally and bicortical locked screws distally; Construct B – Synthes locked plate with alternate unicortical locked screws and cables proximally and bicortical locked screws distally. Construct C – Zimmer cable plate with alternate unicortical non locked screws and cables proximally and bicortical non locked screws distally. Construct D – Zimmer cable plate in same fashion as construct C plus anterior strut allograft secured with cables proximally and distally.

Results: In axial compression, construct D was significantly stiffer compared with all other constructs in the presence of a gap, with no differences between groups without a gap. For lateral bending stiffness, construct D was significantly stiffer than the other groups with and without a gap. In torsional testing, construct D was significantly stiffer than all other constructs in the presence of a gap. With no gap, construct D was significantly stronger than construct B. There were no significant differences between constructs A and B in all testing modalities. Axial load-to-failure ranged from 5561.5 to 6700.2 N. There were no significant differences in axial load to failure.

Conclusions: This study suggests that a single locked plate does not provide the same initial fixation stiffness as a plate-allograft strut construct in the setting of a gapped osteotomy. This may be particularly important in the setting of a comminuted fracture or with bone loss. In these settings, a construct with a lateral plate and an allograft strut placed anteriorly at 90 degrees to the plate, may be optimal.

Purpose: The purpose of this study was to quantify cortical bone screw pullout force and extraction shear stress in synthetic femurs. The use of commercially available synthetic bone analogues has grown increasingly in the literature. They are commonly employed as human femur surrogates for use in biomechanical assessment of orthopaedic fracture fixation devices. However, whether screw purchase is the same in synthetic products as in human bone has not been addressed previously in the literature.

Methods: Three large left adult Third Generation Composite Femurs or 3GCF’s (Model #3306, Pacific Research Labs, Vashon, Washington, USA) were mounted in a test jig that allowed exposure of their cortical mid-shaft area. Standard 3.5 mm (40 mm length) orthopaedic bicortical screws (Synthes, Paoli, PA, USA) were inserted at 5 locations in each specimen along their diaphyseal length and extracted using an Instron test machine. The pullout strengths were recorded and extraction shear stresses were calculated. The tests were repeated on 3 other synthetic femurs using 4.5 mm (40 mm length) bicortical screws. The results were compared to existing adult human cadaveric and animal data from the literature.

Results: For 3.5 mm screws, pullout forces were measured (3.88 to 4.97 kN) and the effective extraction shear stresses calculated (23.70 to 33.99 MPa). The extraction shear stresses were in the range of that found in the literature on human cadaveric femurs and tibias (24.4 to 38.8 MPa). Similarly, for 4.5 mm screws, pullout forces were obtained (5.21 to 7.47 kN) and the extraction shear stresses computed (26.04 to 34.76 MPa). This overlapped with previously published extraction shear stress results for human femurs and tibias (15.9 to 38.9 MPa).

Conclusions: The commercially available 3GCF femurs provide a satisfactory biomechanical analogue to human femurs and tibias at the screw-bone interface during axial screw pullout.

Purpose: Endoprosthetic reconstruction of the distal femur is the preferred approach for patients undergoing resection of bone sarcomas. The traditional How-medica Modular Resection System, using a press-fit stem (HMRS or Kotz prosthesis, Stryker Orthopaedics, Mahwah, New Jersey, USA) has shown good long-term clinical success, but has also been known to incur complications such as stem fracture. The Restoration stem, as a part of the new Global Modular Resection System (GMRS, Stryker Orthopaedics, Mahwah, NJ, USA), is currently proposed for this same application. This stem has a different geometry and provides the advantage of decreased risk of fracture of the component. The goal of this study was to compare the HMRS and Restoration press-fit stems in terms of initial mechanical stability.

Methods: Six matching pairs fresh frozen adult femora were obtained and prepared using a flexible canal reamer and fitted with either a Restoration or HMRS press-fit stem distally. All constructs were mechanically tested in axial compression, lateral bending, and torsion to obtain mechanical stiffness. Torque-to-failure was finally performed to determine the offset force required to clinically fail the specimen by either incurring damage to the femur, the stem, or the femur-stem interface.

Results: Restoration press-fit stems results were: axial stiffness (average=1871.1 N/mm, SD=431.2), lateral stiffness (average=508.0 N/mm, SD=179.6), and torsional stiffness (average=262.3 N/mm, SD=53.2). HMRS stems achieved comparable levels: axial stiffness (average=1867.9 N/mm, SD=392.0), lateral bending stiffness (average=468.5 N/mm, SD=115.3), and torsional stiffness (average=234.9 N/mm, SD=62.4). For torque-to-failure, the applied offset forces on Restoration (average=876.3 N, SD=449.6) and HMRS (aver-age=690.5 N, SD=142.0) stems were similar. There were no statistical differences in performance between the two stem types regarding axial compression (p=0.97), lateral bending (p=0.45), or torsional stiffnesses (p=0.07). Moreover, no differences were detected between the groups when tested in torque-to-failure (p=0.37). The mechanism of torsional failure for all specimens was “spinning” (i.e. surface sliding) at the femur-stem interface. No significant damage was detected to any bones or stem devices.

Conclusions: These results suggest that the Restoration and HMRS press-fit stems may be equivalent clinically in the immediate post-operative situation. Funding: Commerical funding Funding Parties: Stryker Orthopaedics