M.J. Rudert

The effect of incongruity and instability on contact stress directional gradients in human cadaveric ankles

OBJECTIVE Measure incongruity and instability-associated changes in transient contact stress directional gradients in a human cadaveric ankle model. METHODS Seven cadaveric ankles were subjected to quasi-physiologic forces and motion under intact conditions and with a stepoff incongruity of the anterior one-third of the distal tibia. Anterior/posterior forces were modulated to create incongruous specimens that either maintained a stable articulation between the talus and distal tibia or developed gross instability during motion. Real-time contact stresses were measured using a custom-designed ankle stress transducer at 132 Hz. Contact stress data were differentiated using a central-differencing formula to calculate transient contact stress directional gradients over the entire ankle articulation. RESULTS Transient 95th percentile contact stress directional gradient values increased by 30 and 100%, respectively, in stable-incongruous and unstable-incongruous conditions compared to intact conditions. Compared to stable-incongruous conditions, transient contact stress directional gradients increased by 60% in unstable-incongruous conditions. CONCLUSIONS Instability resulted in greater percentage increases in transient contact stress directional gradients compared to incongruity. Pathologic increases in contact stress directional gradients potentially play an important role in the etiology of post-traumatic arthritis.

A clinically realistic large animal model of intra-articular fracture that progresses to post-traumatic osteoarthritis

OBJECTIVE Translation of promising treatments for post-traumatic osteoarthritis (PTOA) to patients with intra-articular fracture (IAF) has been limited by the lack of a realistic large animal model. To address this issue we developed a large animal model of IAF in the distal tibia of Yucatan minipigs and documented the natural progression of this injury. DESIGN Twenty-two fractures were treated using open reduction and internal fixation with either an anatomic reduction or an intentional 2-mm step-off. Pre-operatively, and 3 days, 1, 2, 4, 8, and 12 weeks post-operatively, animals were sedated for synovial fluid draws and radiographs. Limb loading was monitored at the same time points using a Tekscan Walkway. Animals were sacrificed at 12 weeks and the limbs were harvested for histological evaluation. RESULTS All animals achieved bony union by 12 weeks, facilitating nearly complete recovery of the initial 60% decrease in limb loading. TNFα, IL1β, IL6, and IL8 concentrations in the fractured limbs were elevated (P < 0.05) at specific times during the 2 weeks after fracture. Histological cartilage degeneration was more severe in the step-off group (0.0001 < P < 0.27 compared to normal) than in the anatomic reconstruction group (0.27 < P < 0.99 compared to normal). CONCLUSIONS This model replicated key features of a human IAF, including surgical stabilization, inflammatory responses, and progression to osteoarthritic cartilage degeneration, thereby providing a potentially useful model for translating promising treatment options to clinical practice.

An instrumented pendulum system for measuring energy absorption during fracture insult to large animal joints in vivo.

For systematic laboratory studies of bone fractures in general and intra-articular fractures in particular, it is often necessary to control for injury severity. Quantitatively, a parameter of primary interest in that regard is the energy absorbed during the injury event. For this purpose, a novel technique has been developed to measure energy absorption in experimental impaction. The specific application is for fracture insult to porcine hock (tibiotalar) joints in vivo, for which illustrative intra-operative data are reported. The instrumentation allowed for the measurement of the delivered kinetic energy and of the energy passed through the specimen during impaction. The energy absorbed by the specimen was calculated as the difference between those two values. A foam specimen validation study was first performed to compare the energy absorption measurements from the pendulum instrumentation versus the work of indentation performed by an MTS machine. Following validation, the pendulum apparatus was used to measure the energy absorbed during intra-articular fractures created in 14 minipig hock joints in vivo. The foam validation study showed close correspondence between the pendulum-measured energy absorption and MTS-performed work of indentation. In the survival animal series, the energy delivered ranged from 31.5 to 48.3 Js (41.3±4.0, mean±s.d.) and the proportion of energy absorbed to energy delivered ranged from 44.2% to 64.7% (53.6%±4.5%). The foam validation results support the reliability of the energy absorption measure provided by the instrumented pendulum system. Given that a very substantial proportion of delivered energy passed--unabsorbed--through the specimens, the energy absorption measure provided by this novel technique arguably provides better characterization of injury severity than is provided simply by energy delivery.

147 ANKLE CONTACT MECHANICS FOLLOWING FOCAL DEFECT RESURFACING WITH A METALLIC IMPLANT: A COMPUTATIONAL INVESTIGATION

Purpose: We have previously shown that subjects with endstage unilateral hip osteoarthritis (OA) more commonly develop endstage OA of the contralateral knee rather than the ipsilateral knee, and that these subjects have higher dynamic joint loads at the contralateral knee than the ipsilateral knee. Local bone mineral density (BMD) reflects the loading history of that bone; hence, BMD may be a marker of excessive loading of the knee joint as well as of early asymptomatic OA. Here, we evaluated subjects who had unilateral hip OA but who were asymptomatic at their knees, to test the hypothesis that asymmetric loading of the knees induced by unilateral hip OA results in elevated BMD at the medial tibial plateau of the contralateral knee compared with the ipsilateral knee, and that these BMD asymmetries correlate with dynamic joint loading at the knees. Methods: Fifty subjects with symptomatic unilateral hip OA and asymptomatic knees were evaluated. Subjects had moderate to severe radiographic unilateral hip OA using the Kellgren Lawrence (KL) grading system. Subjects were asymptomatic at the knees (WOMAC pain during walking <20 mm out of 100 mm). Subjects underwent DXA scanning of bilateral knees and these scans were evaluated in a blinded manner by a trained investigator using a previously validated method. The BMD of the medial and lateral regions of the tibial plateau and the distal tibial shaft were measured in each knee. Subjects also underwent gait analyses using an optoelectronic camera system and multicomponent force plate. Inverse dynamics were used to calculate dynamic joint loads and the peak external knee adduction moment, a validated marker of medial compartment knee loading, was used as the primary load parameter. Paired t-tests were used to evaluate differences in BMD and loading between the knees and Spearman correlations were used to evaluate correlations between BMD and loading. p<0.05 was considered significant. Results: Bone mineral density was significantly increased at the contralateral medial tibial plateau compared with the ipsilateral medial tibial plateau (0.912g/cm2±0.208 vs 0.869g/cm2±0.196 p=0.040). Furthermore, a direct correlation was found between the medial knee load (peak external knee adduction moment) and BMD at the contralateral medial knee (rho=0.381, p=0.008). No significant differences were noted for BMD at the lateral compartments of the two knees. Conclusions: This study demonstrates that unilateral hip OA is associated with increased BMD at the contralateral medial knee when compared with the ipsilateral medial knee, that BMD alterations are directly correlated with loading alterations at the OA-predisposed knee (contralateral knee), and that these events occur even in asymptomatic, clinically uninvolved knees. These findings suggest that BMD alterations may be a surrogate marker for joint loading and OA progression, even in asymptomatic subjects. Although further investigation is necessary to delineate causal relationships, BMD may be a useful tool to follow structural progression in longitudinal OA studies. 147

121 SPATIAL CORRELATIONS BETWEEN LOCAL IMPACT STRESS AND CELL DEATH DISTRIBUTIONS

Conclusions: This study is to the authors’ knowledge the first to identify risk for incident symptomatic knee OA by subject-specific biomechanical modeling. Using DEA was an efficient means of estimating subjectspecific articular contact. The presence of differences in estimated contact stress by imaging 15 months prior to diagnosis suggests a possible biomechanical mechanism for the development of symptomatic knee OA.