Locked plate constructs are not necessarily stiffer than nonlocked constructs: A biomechanical investigation of locked versus nonlocked diaphyseal fixation in a human cadaveric model of nonosteoporotic and osteoporotic distal femoral fractures.

Objectives: The objective of this study was to compare the biomechanical properties of locked and nonlocked diaphyseal fixation in a model of distal femur fractures using osteoporotic and nonosteoporotic human cadaveric bone. Methods: A supracondylar osteotomy was created to mimic a fracture (OTA/AO 33A3) in osteoporotic (n = 4) and nonosteoporotic (n = 5) cadaveric distal femurs. The left and right femurs of each pair were instrumented with a distal femoral locking plate and randomly assigned to have diaphyseal fixation with either locked or nonlocked screws. The construct was cyclically axially loaded, and construct stiffness and load to failure were evaluated. Results: In osteoporotic bone, locked constructs were more stiff than nonlocked constructs (mean 143 vs. 98 N/mm when all time points combined, P < 0.001). However, in nonosteoporotic bone, locked constructs were less stiff than nonlocked constructs (mean 155 N/mm vs. 185 N/mm when all time points combined, P < 0.001). In osteoporotic bone, the average load to failure was greater in the locked group than in the nonlocked group (mean 1159 vs. 991 N, P = 0.01). In nonosteoporotic bone, the average load to failure was greater for the nonlocked group (mean 1348 N vs. 1214 N, P = 0.02). Bone mineral density was highly correlated with maximal load to failure (R2 = 0.92, P = 0.001) and stiffness (R2 = 0.78, P = 0.002) in nonlocked constructs but not in locked constructs. Conclusions: Contrary to popular belief, locked plating constructs are not necessarily stiffer than nonlocked constructs. In healthy nonosteoporotic bone, locked diaphyseal fixation does not provide a stiffer construct than nonlocked fixation. Bone quality has a profound influence on the stiffness of nonlocked (but not locked) constructs in distal femur fractures.

Locking Hole Inserts: Effect of Insertion Torque on Fatigue Performance and Insert Loosening in Locking Plates.

OBJECTIVES: To evaluate the effect of locking hole inserts (LHIs) and their insertion torque on locking plate fatigue life. METHODS: Eighteen standard 3.5-mm locking plates were instrumented with LHIs (Smith & Nephew, Memphis, TN) of 1.70 or 3.96 Nm insertion torque, or without LHIs, whereas eleven 4.5-mm locking plates were instrumented with LHIs at 3.96 Nm insertion torque or without LHIs. Plates were cyclically loaded to failure (ie, plate fracture) in four-point bending. Number of cycles to plate failure were measured. RESULTS: The 3.5-mm plates with 1.70 Nm LHI insertion torque had a 52% increase in cycles to failure compared with plates without LHIs (114,300 ± 23,680 vs. 75,487 ± 15,746 cycles; P = 0.01). Increasing insertion torque to 3.96 Nm led to a further increase of 36% in fatigue life (155,177 ± 32,493 cycles; P = 0.02) and a 106% increase compared with plates without LHIs (P = 0.001). The 4.5-mm plates with 3.96 Nm insertion torque had a 48% increase in cycles to failure when compared with plates without LHIs (74,369 ± 10,181 vs. 50,214 ± 5544 cycles; P = 0.001). CONCLUSIONS: LHIs significantly extend plate fatigue length, which would be advantageous in the setting of delayed fracture healing. We recommend the use of LHIs in round locking holes over bony gaps whenever possible; however, we recognize that these findings are limited to implants manufactured by Smith & Nephew.

Characterization of Femoral Head Taper Corrosion Features Using a 22-Year Retrieval Database.

BACKGROUND: Modularity in total hip arthroplasty has been used for decades with great success, but new findings regarding corrosion artifacts have caused a resurgence in tapered junction research. Mechanically assisted crevice corrosion (MACC) is thought to be the mechanism by which corrosive attack occurs. Myriad multi-factorial variables are known to influence the susceptibility of a modular taper junction to MACC. Some of these variables are design and manufacture related and others can be controlled by the surgeon. QUESTIONS/PURPOSES: This study was performed to assess a 22-year retrieval database to determine if correlations exist between severity of corrosion artifacts and head size, time in vivo, head offset, or head material. Secondarily, the agreement of visual and semi-quantitative scoring methods was assessed using the retrieved components. METHODS: A total of 210 femoral head tapers were scored and heads receiving high scores were measured to quantify material loss due to MACC. RESULTS: Increased head size and increased time in vivo did not correlate to higher corrosion scores. Contrarily, there were differences in corrosion scores based on femoral head offset and material. Deviations away from a neutral offset (where neutral is defined as the alignment of femoral head center and stem taper gage point) resulted in higher scores. Cobalt-chromium-molybdenum heads were associated with higher corrosion scores and higher material loss as compared to oxidized zirconium heads. CONCLUSION: Reducing the moment arm at the head-neck junction and choosing a more inert material appears to provide greater resistance to corrosion.

Mechanical effects of off-axis insertion of locking screws: should we do it?

OBJECTIVES: The purposes of this study were to evaluate the cantilevered bending strength and failure modes of locking screws inserted at various angles in a plate with fully circumferential threaded holes. As an additional measure, the amount of screw head prominence at these angles was also assessed. METHODS: Standard 3.5-mm locking screws were inserted into round fully circumferential threaded holes through a standard straight 3.5-mm locking plate at various angles. The achieved angle of insertion and its prominence protruding from the far-bone side of the plate was measured using an optical luminescence technique. Each screw was then loaded at a constant rate until failure in a cantilevered bending scenario. The maximum cantilevered bending strength was measured, and the moment at failure was calculated. RESULTS: There was a positive correlation between increasing insertion angle and increasing prominence; a higher screw insertion angle yielded greater prominence. Prominence values ranged from negligible to 2 mm. As screw insertion angle increased, the bending moment at failure decreased. Screws inserted to 3 degrees or below primarily failed through screw deformation at the minor diameter below the head, whereas screws inserted to greater than 3 degrees primarily failed through locking mechanism disengagement. CONCLUSIONS: These findings indicate that cross threading may not be biomechanically advantageous and may change screw mode of failure. Based on these findings, screws inserted to 3 degrees or higher would reduce the bending moment at failure to approximately 50% of an orthogonally inserted screw.

Can we trust ex vivo mechanical testing of fresh--frozen cadaveric specimens? The effect of postfreezing delays.

BACKGROUND: Because embalming has been demonstrated to decrease the mechanical integrity of bone, most investigators favor fresh-frozen specimens for biomechanical evaluation. However, little is known about how the integrity of fresh--frozen specimens may change during biomechanical testing or may be affected by standard practices in testing. OBJECTIVE: The purpose of this study was to evaluate how the time after removal from a freezer may affect the mechanical properties of fresh--frozen diaphyseal bone. METHODS: Matched pairs of nonosteoporotic fresh--frozen human cadaveric femora were thawed before instrumentation with bicortical screws. Matched femora were reserved for either control or delayed use. Each specimen received standard diaphyseal bicortical screws (six or more in each group). At specified time points, screws were axially pulled out following the guidelines of ASTM F543-07. Test groups were stored in air (21 ± 0.5°C) for 16, 50, or 90 hours. In the control group, screws were pulled out at 16 hours, which corresponds to the minimum elapsed time for specimen thawing, instrumentation, potting, and biomechanical test initiation. This represents the baseline mechanical properties of the fresh--frozen bone at the inception of any biomechanical test. The 90-hour group corresponds to the time needed to cycle a construct 300,000 times at a physiological test frequency of 1 Hz. This corresponds approximately to 2 to 4 months of in vivo loading. A midpoint of 50 hours was also tested, representing approximately 180,000 cycles. RESULTS: Failure for all specimens occurred as a result of bone failure at the screw-to-bone interface. There was a decrease in screw pullout strength as exposure time in air increased. The 50-hour test group showed a 9% decrease in screw pullout strength as compared with the 16-hour control group (P = 0.81). However, the 90-hour test group showed a 30% decrease in screw pullout strength as compared with the 16-hour control group (P = 0.04). CONCLUSION: This study indicates that when using fresh-frozen cadaveric bone in biomechanical tests to simulate the orthopaedic clinical setting, specimen exposure time should be considered. The timing of testing should be kept constant between specimens to allow for a proper comparison. Furthermore, for fresh--frozen cadavers, the physical properties of bone may be detrimentally affected in biomechanical testing that exceeds the 50-hour time point after removal from the freezer.

A comparison of screw insertion torque and pullout strength.

PURPOSE: Pullout strength of screws is a parameter used to evaluate plate screw fixation strength. However, screw fixation strength may be more closely related to its ability to generate sufficient insertion because stable nonlocked plate-screw fracture fixation requires sufficient compression between plate and bone such that no motion occurs between the plate and bone under physiological loads. Compression is generated by tightening of screws. In osteoporotic cancellous bone, sufficient screw insertion torque may not be generated before screw stripping. The effect of screw thread pitch on generation of maximum insertion torque (MIT) and pullout strength (POS) was investigated in an osteoporotic cancellous bone model and the relationship between MIT and POS was analyzed. METHODS: Stainless steel screws with constant major (5.0 mm) and minor (2.7 mm) diameters but with varying thread pitches (1, 1.2, 1.5, 1.6, and 1.75 mm) were tested for MIT and POS in a validated osteoporotic surrogate for cancellous bone (density of 160 kg/m(3) [10 lbs/ft(3)]). MIT was measured with a torque-measuring hex driver for screws inserted through a one-third tubular plate. POS was measured after insertion of screws to a depth of 20 mm based on the Standard Specification and Test Methods for Metallic Medical Bone Screws (ASTM F 543-07). Five screws were tested for each failure mode and screw design. The relationship between MIT and compressive force between the plate and bone surrogate was evaluated using pressure-sensitive film. RESULTS: There was a significant difference in mean MIT based on screw pitch (P < 0.0001), whereas POS did not show statistically significant differences among the different screw pitches (P = 0.052). Small screw pitches (1.0 mm and 1.2 mm) had lower MIT and were distinguished from large pitches (1.5 mm, 1.6 mm, and the 1.75 mm) with higher MIT. For POS, only the 1-mm and 1.6-mm pitch screws were found to be different from each other. Linear regression analysis of MIT revealed a moderate correlation to the screw pitch (R(2) = 0.67, P < 0.0001), whereas the analysis of POS suggested no correlation to the screw pitch (R(2) = 0.28, P = 0.006). Pearson correlation analysis indicated no correlation between MIT and POS (P = 0.069, r = -0.37). A linear relationship of increased compression between the plate and bone surrogate was found for increasing screw torque (R(2) = 0.97). CONCLUSIONS: These results indicate that the ability of different screw designs to generate high screw insertion torque in a model of osteoporotic cancellous bone is unrelated to their pullout strength. Therefore, extrapolation of results for POS to identify optimal screw design for osteoporotic bone may not be valid. Screw designs that optimize MIT should be sought for fixation in osteoporotic bone.

Stress corrosion cracking of an aluminum alloy used in external fixation devices.

Treatment for compound and/or comminuted fractures is frequently accomplished via external fixation. To achieve stability, the compositions of external fixators generally include aluminum alloy components due to their high strength-to-weight ratios. These alloys are particularly susceptible to corrosion in chloride environments. There have been several clinical cases of fixator failure in which corrosion was cited as a potential mechanism. The aim of this study was to evaluate the effects of physiological environments on the corrosion susceptibility of aluminum 7075-T6, since it is used in orthopedic external fixation devices. Electrochemical corrosion curves and alternate immersion stress corrosion cracking tests indicated aluminum 7075-T6 is susceptible to corrosive attack when placed in physiological environments. Pit initiated stress corrosion cracking was the primary form of alloy corrosion, and subsequent fracture, in this study. Anodization of the alloy provided a protective layer, but also caused a decrease in passivity ranges. These data suggest that once the anodization layer is disrupted, accelerated corrosion processes occur.