Search

Background: The Taylor Spatial Frame™ (TSF) is a versatile variant of the traditional Ilizarov circular fixator. Although in widespread use, little comparative data exist to quantify the biomechanical effect of substituting the tried-and-tested Ilizarov construct for the TSF hexapod system.Questions/purposes: This study was designed to investigate the mechanical properties of the TSF system under physiologic loads, with and without the addition of a simulated bone model, with comparison to the standard Ilizarov frame.Methods: The mechanical behaviors of three identical four-ring TSF and Ilizarov constructs were tested under levels of axial compression, bending, and rotational torque to simulate loading during normal gait. An acrylic-pipe fracture model subsequently was mounted, using fine wires and 5 mm half pins, and the testing was repeated. Load-deformation curves, and so rigidity, for each construct were calculated, with statistical comparisons performed using paired t-tests.Results: Under axial loading, the TSF was found to be less rigid than the Ilizarov frame (645 ± 57 N/mm versus 1269 ± 256 N/mm; mean difference, 623 N/mm; 95% CI, 438.3-808.5 N/mm; p < 0.001), but more rigid under bending and torsional loads (bending: 42 ± 9 Nm/degree versus 78 ± 13 Nm/degree; mean difference, 37 Nm/degree; 95% CI, 25.0-47.9 Nm/degree; p < 0.001; torsion: 16 ± 2 Nm/degree versus 5 ± 0.35 Nm/degree; mean difference, 11 Nm/degree; 95% CI, 9.5-12.2 Nm/degree; p < 0.001). On mounting the bone models, these relationships broadly remained in the half-pin and fine-wire groups, however the half-pin constructs were universally more rigid than those using fine wires. This effect resulted in the TSF, using half pins, showing no difference in axial rigidity to the fine-wire Ilizarov (107 ± 3 N/mm versus 107 ± 4 N/mm; mean difference, 0.05 N/mm; 95% CI, -6.99 to 7.1 N/mm; p > 0.999), while retaining greater bending and torsional rigidity. Throughout testing, a small amount of laxity was observed in the TSF construct on either side of neutral loading, amounting to 0.72 mm (±0.37 mm) for a change in loading between -10 N and 10 N axial load, and which persisted with the addition of the synthetic fracture model.Conclusions: This study broadly shows the TSF construct to generate lower axial rigidity, but greater bending and torsional rigidity, when compared with the Ilizarov frame, under physiologic loads. The anecdotally described laxity in the TSF hexapod strut system was shown in vitro, but only at low levels of loading around neutral. It also was shown that the increased stiffness generated by use of half pins produced a TSF construct replicating the axial rigidity of a fine-wire Ilizarov frame, for which much evidence of good clinical and radiologic outcomes exist, while providing greater rigidity and so improved resistance to potentially detrimental bending and rotational shear loads.Clinical Relevance: If replicated in the clinical setting, these findings suggest that when using the TSF, care should be taken to minimize the observed laxity around neutral with appropriate preloading of the construct, but that its use may produce constructs better able to resist bending and torsional loading, although with lower axial rigidity. Use of half pins in a TSF construct however may replicate the axial mechanical behavior of an Ilizarov construct, which is thought to be conducive to bone healing. [ABSTRACT FROM AUTHOR]

Background: Autograft stabilization uses free semitendinosus tendon grafts to anatomically reconstruct the anterior talofibular ligament. Study aims were to evaluate the biomechanical properties of Mitek GII anchors compared with the Arthrex BioTenodesis Screw for free tendon reconstruction of the anterior talofibular ligament. Null Hypothesis: There are no differences in load to failure and percentage specimen elongation at failure between the 2 methods. Study Design: Controlled laboratory study using porcine models. Methods: Sixty porcine tendon constructs were failure tested. Re-creating the pull of the anterior talofibular ligament, loads were applied at 700 to the bones. Thirty-six tendons were fixed to porcine tall and tested using a single pull to failure; 10 were secured with anchors and No. 2 Ethibond, 10 with anchors and FiberWire, 10 with screws and Fiberwire, and 6 with partially gripped screws. Cyclic preloading was conducted on 6 tendons fixed by anchors and on 6 tendons fixed by screws before failure testing. Two groups of 6 components fixed to the fibula were also tested. Results: The talus single-pull anchor group produced a mean load of 114 N and elongation of 37% at failure. The talus single- pull screw group produced a mean load of 227 N and elongation of 22% at failure (P < .05). Cyclic preloading at 65% failure load before failure testing produced increases in load and decreases in elongation at failure. Partially gripped screws produced a load of 133 N and elongation of 30% at failure. The fibula model produced significant increases in load to failure for both. The human anterior talofibular ligament has loads of 139 N at failure with instability occurring at 20% elongation. Conclusions: Interference screw fixation produced significantly greater failure strength and less elongation at failure than bone anchors. Clinical Relevance: The improved biomechanics of interference screws suggests that these may be more suited to in vivo reconstruction of the anterior talofibular ligament than are bone anchors. [ABSTRACT FROM AUTHOR]

• Joint reaction force on its own is not sufficient for assessment of contact mechanics. • Timing of load varies significantly with activity. • Duration of load varies significantly with activity. • Localised changes in velocity and acceleration occur within the contact area. • A virtual hip joint model can provide detailed characteristics for contact analysis. The paper reports the characteristics of joint forces for 9 activities in 18 normal healthy subjects. Activities included Walk, Walk Turn, Stand to Sit, Sit to Stand, Squat, Stand Reach, Kneel Reach, Lunge, and Golf Swing. Within the cohort ∼30% variability occurred in the manner in which each activity was completed. Within the activities the average maximum load characteristics varied in magnitude (0.5–6.4 ρBWT) and also in duration (0.96–5.89 s.) when compared to walking (3.1 ρBWT,1.1 s.). The corresponding impulse ranged from 1.6 during the Walk to 6.7 ρ.BWT.s for the Golf Swing. As high loads with low sliding velocities have been shown in the literature to be damaging to the tribology of compliant contact surfaces the findings are postulated by the authors to be specifically important for the pre-clinical testing of cartilage substitutional materials. Note: Force was normalized to body weight (ρBWT) throughout the study. [ABSTRACT FROM AUTHOR]

Background Symptomatic leg length inequality accounts for 8.7% of total hip replacement related claims made against the UK National Health Service Litigation authority. It has not been established whether symptomatic leg length inequality patients following total hip replacement have abnormal hip kinetics during gait. Methods Hip kinetics in 15 unilateral total hip replacement patients with symptomatic leg length inequality during gait was determined through multibody dynamics and compared to 15 native hip healthy controls and 15 ‘successful’ asymptomatic unilateral total hip replacement patients. Finding More significant differences from normal were found in symptomatic leg length inequality patients than in asymptomatic total hip replacement patients. The leg length inequality patients had altered functions defined by lower gait velocity, reduced stride length, reduced ground reaction force, decreased hip range of motion, reduced hip moment and less dynamic hip force with a 24% lower heel-strike peak, 66% higher mid-stance trough and 37% lower toe-off peak. Greater asymmetry in hip contact force was also observed in leg length inequality patients. Interpretation These gait adaptions may affect the function of the implant and other healthy joints in symptomatic leg length inequality patients. This study provides important information for the musculoskeletal function and rehabilitation of symptomatic leg length inequality patients. [ABSTRACT FROM AUTHOR]

Background Preclinical durability testing of hip replacement implants is standardised by ISO-14242-1 (2002) which is based on historical inverse dynamics analysis using data obtained from a small sample of normal healthy individuals. It has not been established whether loading cycles derived from normal healthy individuals are representative of loading cycles occurring in patients following total hip replacement. Methods Hip joint kinematics and hip contact forces derived from multibody modelling of forces during normal walking were obtained for 15 asymptomatic total hip replacement patients and compared to 38 normal healthy individuals and to the ISO standard for pre-clinical testing. Findings Hip kinematics in the total hip replacement patients were comparable to the ISO data and the hip contact force in the normal healthy group was also comparable to the ISO cycles. Hip contact forces derived from the asymptomatic total hip replacement patients were comparable for the first part of the stance period but exhibited 30% lower peak loads at toe-off. Interpretation Although the ISO standard provides a representative kinematic cycle, the findings call into question whether the hip joint contact forces in the ISO standard are representative of those occurring in the joint following total hip replacement. [ABSTRACT FROM AUTHOR]

A calcar collar may reduce risk of periprosthetic fracture of the femur, through collar contact. We estimated the effect of collar contact on periprosthetic fracture mechanics using a collared fully coated cementless femoral stem and then estimated the effect of initial calcar-collar separation on the likelihood of collar contact. Three groups of six composite left femurs with increasing calcar-collar separation in each group, underwent periprosthetic fracture simulation in a materials testing machine. Fracture torque and rotational displacement were measured and torsional stiffness and rotational work prior to fracture were estimated. Calcar collar contact prior to fracture was identified using high speed camera footage. Where calcar-collar contact occurred fracture torque was greater (47.33 [41.03 to 50.45] Nm versus 38.26 [33.70 to 43.60] Nm, p = 0.05), Rotational displacement was less (16.6 [15.5 to 22.3] degrees versus 21.2 [18.9 to 28.1] degrees, p = 0.07), torsional stiffness was greater (151.38 [123.04 to 160.42] rad.Nm−1 versus 96.86 [84.65 to 112.98] rad.Nm−1, p < 0.01) and rotational work was similar (5.88 [4.67, 6.90] J versus 5.31 [4.40, 6.56] J, p = 0.6). Odds ratio (OR) of not achieving collar contact (95% confidence interval) increased 3.8 fold (95% CI 1.6 to 30.2, p < 0.05) for each millimetre of separation in the regression model. 95% chance of collar contact was associated with a separation of 1 mm or less. Surgeons should reduce calcar-collar separation at stem implantation to a maximum of 1 mm to increase the chance of calcar-collar contact during injury and reduce the risk of early post-operative femoral fracture. • A medial calcar collar is associated with a decreased risk of periprosthetic fracture of the femur. • Calcar-collar contact is crucial to minimising the risk of early periprosthetic femoral fracture. • Surgeons should achieve a calcar-collar separation of 1 mm or less to ensure Calcar-collar contact. [ABSTRACT FROM AUTHOR]