A biomechanical comparison of static versus dynamic lag screw modes for cephalomedullary nails used to fix unstable peritrochanteric fractures.
ABSTRACT The gamma nail has an option to statically lock its lag screw (static mode) or to allow its 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 lag screw modes for a cephalomedullary nail used to fix an unstable peritrochanteric fracture.
Unstable four-part peritrochanteric fractures were created in 30 synthetic femurs and fixed with Long Gamma 3 Nails. Mechanical tests were conducted for axial, lateral, and torsional stiffness with intact femurs, femur-nail constructs with static lag screw mode,and femur-nail constructs with dynamic lag screw mode. A paired Student's t test was used for all statistical comparisons between test groups.
Axial and torsional stiffness of intact femurs was significantly greater than femur-nail constructs (p < 0.01 all comparisons),whereas lateral stiffness was significantly less (p < 0.01 all comparisons). Axial stiffness of the femur-nail construct was significantly greater (p < 0.01) in static mode (484.3 N/mm 80.2 N/mm) than in dynamic mode (424.1 N/mm 78.0 N/mm).Lateral stiffness was significantly greater (p < 0.01) in static mode (113.9 N/mm 8.4 N/mm) than in dynamic mode (109.5N/mm 8.8 N/mm). Torsional stiffness was significantly greater (p = 0.02) in dynamic mode (114.5 N/mm 28.2 N/mm) than in static mode (111.7 N/mm 27.0 N/mm).
There is a 60 N/mm (12.4%) reduction in axial stiffness when the lag screw is in dynamic mode. Given the statistically significant reduction in axial and lateral stiffness with use of the dynamic mode, static lag screw mode should be further explored clinically for treatment of unstable peritrochanteric fractures.
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ABSTRACT: 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[middle dot]mm, 157.8 ± 16.1 N[middle dot]mm, and 140.2 ± 16.4 N[middle dot]mm) and artificial femurs (67.2 ± 8.4 N[middle dot]mm, 61.0 ± 2.9 N[middle dot]mm, and 53.3 ± 2.9 N[middle dot]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.Journal of Biomechanical Engineering 12/2012; 134(12):124503. · 1.52 Impact Factor
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ABSTRACT: OBJECTIVES:: The femur is the most common long bone affected by cancerous metastasis. Femoral tumor defects are known to induce pain and functional impairment in patients. Although prior studies exist evaluating the clinical and biomechanical effect of tumor defect size, no biomechanical studies have experimentally examined the risk of pathological fracture with respect to the anterior, posterior, medial, and lateral surfaces on which a proximal tumor defect is located on the femur. METHODS:: Circular tumor-like defects of 40-mm diameter were created proximally in the subtrochanteric region on the Anterior (n=5), Posterior (n=5), Medial (n=5), and Lateral (n=5) sides of 20 synthetic femurs. Intact femurs served as a control group (n=4). Femurs were tested for lateral, "offset" torsional, and axial stiffness, as well as axial strength. RESULTS:: Lateral stiffnesses (range, 121 to 162 N/mm) yielded no differences between groups (p = 0.069). "Offset" torsional stiffnesses (range, 135 to 188 N/mm) demonstrated that the Medial group was less stiff than the Intact, Anterior, and Lateral groups (p ≤ 0.012). Axial stiffnesses (range, 1057 to 1993 N/mm) showed that the Medial group was less stiff than the Intact group (p = 0.006). Axial strengths (range, 3250 to 6590 N) for the Medial group were lower than Anterior (p = 0.001) and Posterior (p = 0.001) specimens, while the Lateral group had a lower strength than Anterior specimens (p = 0.019). No other statistical differences were noted. Axial failure of Medial and Lateral specimens involved the tumor-like defect in 100% of cases, whereas 100% of Intact femurs and 80% of Anterior and Posterior femur groups failed only through the neck. CONCLUSIONS:: In 2 of 3 test modes, the Medial tumor-like defect group resulted in statistically lower stiffness values compared to Intact femurs and had lower strength than Anterior and Posterior groups in axial failure.Journal of orthopaedic trauma 12/2012; · 1.78 Impact Factor