Elevated Contact Stress Research Articles

Experimental and finite element analysis of surface asperity geometry during the running-in phase of rolling contact

This paper presents the experimental and finite element analysis of surface asperity geometry during the running-in phase of rolling contact. Previous research efforts typically relied on simulating various shapes of asperity geometry to elucidate rough surface characteristics. However, this approximation did not accurately represent the actual surface asperities. In this study, a rolling contact rig was fabricated, and periodic surface scans were utilized to track the deformation of roller surface asperities. The experimental findings reveal a notable 69% reduction in surface roughness throughout the running-in phase, alongside showcasing the deformation of asperity geometry. Subsequently, a simulation model is developed using data derived from these experiments. Stress analysis conducted on individual and multiple asperities illustrates a decrease in contact stress over time, indicating a transition in contact behavior from plastic to elastic. Furthermore, simulations involving multiple asperities demonstrate an expansion in contact length as roughness diminishes with increasing cycles. Initially, only the highest peaks of asperities make contact, resulting in elevated contact stress. However, as rolling cycles progress, a greater number of asperities come into contact, leading to a more uniform distribution of load. Notably, the more prominent asperities endure significant contact stress and deformation compared to their smaller counterparts.

Wear-induced microstructural evolution in CoCrNi-based high-entropy alloys at cryogenic temperature

Elucidating the tribological behaviors of CoCrNi-based high-entropy alloys (HEAs) at cryogenic temperatures is crucial for their practical application. This work uncovers the correlation between the wear-induced microstructural evolutions and tribological properties of single-phase CoCrFeNiMn HEA and heterogeneous CoCrFeNiAl HEA upon two contact conditions at 173 K. When CoCrFeNiMn HEA matches with GCr15 rather than Si3N4, low contact stress and high flash temperature suppress the inhomogeneous deformation initiated by martensitic transformation and in turn create a gradient nanograined tribo-layer activated by high-density dislocations and nano-scale deformation twins, which progressively accommodates the frictional strain and reduces wear by an order of magnitude. Instead, the tribological properties of the CoCrFeNiAl HEA sliding against Si3N4 are slightly superior to those sliding against GCr15, owing to the elevated contact stress and reduced flash temperature thicken the protective recrystallized nanocrystalline tribo-layer. Our findings offer guidance for optimizing the tribological properties of HEAs in cryogenic environments.

Providing a computationally derived, mechanically optimised target correction during preoperative planning can improve joint contact mechanics of hip dysplasia treated with periacetabular osteotomy.

Preoperative identification of acetabular corrections that optimally improve joint stability and reduce elevated contact stresses could further reduce osteoarthritis progression in patients with hip dysplasia who are treated with periacetabular osteotomy (PAO). The purpose of this study was to investigate how providing patient-specific, mechanically optimal acetabular reorientations to the surgeon during preoperative planning affected the surgically achieved correction. Preoperative CT scans were used to create patient-specific hip models for 6 patients scheduled for PAO. A simulated acetabular fragment was extracted from the preoperative pelvis model and computationally rotated to simulate candidate acetabular reorientations. For each candidate, discrete element analysis was used to compute contact stresses during walking, which were summed over the gait cycle and scaled by patient age to obtain chronic contact stress-time exposure. The ideal patient-specific reorientation was identified using a cost function that balances minimising chronic stress exposures and achieving surgically acceptable acetabular coverage angles. The optimal reorientation angles and associated contact mechanics were provided to the surgeon preoperatively. After PAO was performed, a model of the surgically achieved correction was created from a postoperative CT scan. Radiographic coverage and contact mechanics were compared between preoperative, optimal, and surgically achieved orientations. While surgically achieved reorientations were not significantly different from optimal reorientations in radiographically measured lateral (p = 0.094) or anterior (p = 0.063) coverage, surgically achieved reorientations had significantly (p = 0.031) reduced total contact area compared to optimal reorientations. The difference in lateral coverage and peak chronic exposure between surgically achieved and optimal reorientations decreased with increasing surgeon experience using the models (R² = 0.758, R2 = 0.630, respectively). Providing hip surgeons with a patient-specific, computationally optimal reorientation during preoperative planning may improve contact mechanics after PAO, which may help reduce osteoarthritis progression in patients with hip dysplasia.

Chronically elevated contact stress exposure correlates with intra-articular cartilage degeneration in patients with concurrent acetabular dysplasia and femoroacetabular impingement.

Hip dysplasia is known to lead to premature osteoarthritis. Computational models of joint mechanics have documented elevated contact stresses in dysplastic hips, but elevated stress has not been directly associated with regional cartilage degeneration. The purpose of this study was to determine if a relationship exists between elevated contact stress and intra-articular cartilage damage in patients with symptomatic dysplasia and femoroacetabular impingement. Discrete element analysis was used to compute hip contact stresses during the stance phase of walking gait for 15 patients diagnosed with acetabular dysplasia and femoral head-neck offset deformity. Contact stresses were summed over the duration of the walking gait cycle and then scaled by patient age to obtain a measure of chronic cartilage contact stress exposure. Linear regression analysis was used to evaluate the relationship between contact stress exposure and cartilage damage in each of six acetabular subregions that had been evaluated arthroscopically for cartilage damage at the time of surgical intervention. A significant correlation (R2 = 0.423, p < 0.001) was identified between chondromalacia grade and chronic stress-time exposure above both a 1 MPa damage threshold and a 2 MPa-years accumulated damage threshold. Furthermore, an over-exposure threshold of 15% regional contact area exceeding the 1 and 2 MPa-years threshold values resulted in correct identification of cartilage damage in 83.3% (55/66) of the acetabular subregions loaded during gait. These results suggest corrective surgery to alleviate impingement and reduce chronic contact stress exposures below these damage-inducing thresholds could mitigate further cartilage damage in patients with hip dysplasia.

Joint contact stress improves in dysplastic hips after periacetabular osteotomy but remains higher than in normal hips.

The purpose of this study was to use computational modeling to determine if surgical correction of hip dysplasia restores hip contact mechanics to those of asymptomatic, radiographically normal hips. Discrete element analysis (DEA) was used to compute joint contact stresses during the stance phase of normal walking gait for 10 individuals with radiographically normal, asymptomatic hips and 10 age- and weight-matched patients with acetabular dysplasia who underwent periacetabular osteotomy (PAO). Mean and peak contact stresses were higher (p < 0.001 and p = 0.036, respectively) in the dysplastic hips than in the matched normal hips. PAO normalised standard radiographic measurements and medialised the location of computed contact stress within the joint. Mean contact stress computed in dysplastic hips throughout the stance phase of gait (median 5.5 MPa, [IQR 3.9-6.1 MPa]) did not significantly decrease after PAO (3.7 MPa, [IQR 3.2-4.8]; p = 0.109) and remained significantly (p < 0.001) elevated compared to radiographically normal hips (2.4 MPa, [IQR 2.2-2.8 MPa]). Peak contact stress demonstrated a similar trend. Joint contact area during the stance phase of gait in the dysplastic hips increased significantly (p = 0.036) after PAO from 395 mm2 (IQR 378-496 mm2) to 595 mm2 (IQR 474-660 mm2), but remained significantly smaller (p = 0.001) than that for radiographically normal hips (median 1120 mm2, IQR 853-1444 mm2). While contact mechanics in dysplastic hips more closely resembled those of normal hips after PAO, the elevated contact stresses and smaller contact areas remaining after PAO indicate ongoing mechanical abnormalities should be expected even after radiographically successful surgical correction.

Elevated contact stress after acetabular fracture correlates with development of radiographic OA

Purpose: Elevated contact stress following intra-articular fracture has been linked to subsequent development of post-traumatic osteoarthritis (PTOA). This relationship has been well established in the ankle, and elevated contact stresses in the hip have been tied to osteoarthritic changes. However, changes in hip contact stress due to articular fracture have not been well studied. Additionally, though elevated hip contact stresses due to residual incongruity have been documented in in vitro studies, to our knowledge, they have never been studied in a series of clinical fracture cases. Therefore, the present study utilizes discrete element analysis (DEA) to investigate changes in contact stress that may occur in patients following surgical reduction of their intra-articular acetabular fractures and the relationship between elevated hip contact stress and radiographic OA outcome in these patients. Methods: Under IRB approval, post-operative CT scans from 10 patients with surgically reduced acetabular fractures were obtained. Five of the cases were considered to not have progressed to OA (KL grade < 2, No OA group), and the remaining five were considered to have developed OA (KL ≥ 2, OA group). Femoral and pelvic anatomy for each patient was segmented from CT using a semi-automated program developed in MATLAB (Mathworks, Natick, MA). Each patient model was smoothed, aligned to an appropriate coordinate system, and moved through a series of loadings drawn from gait data collected using an instrumented total hip. Forces applied to each model were scaled based on the patient’s body weight. DEA was used to compute contact stress during the stance phase of gait. For each patient, the maximum contact stress that developed at any time during the stance phase of gait was compared to their KL grade. The contact stress patterns for these two different groups of acetabular fracture patients were also compared over the full stance phase of gait to DEA-computed contact stresses in 5 trauma patients without hip pain or injury. Results: The maximum contact stress for the OA group was 10.4 MPa, which was significantly (P < 0.001) higher than that for the No OA group (7.2 MPa) or for the patients with normal hips (8.9 MPa, Figure 1). While there was no significant difference in maximum contact stress between the No OA group and the patients with normal hips, the No OA group did have lower contact stress near heel-strike and higher contact stress near toe-off (Figure 1). There was a positive correlation between the maximum contact stress and KL grade (R2 = 0.546, Figure 2).View Large Image Figure ViewerDownload Hi-res image Download (PPT) Conclusions: The acetabular fracture patients who developed OA had significantly higher contact stress than the fracture patients who did not and the patients with normal hips. This suggests that exposure to abnormally high contact stress in the hip leads to the development of OA. The lack of a significant difference in contact stress between the fracture patients who did not develop OA and the patients with normal hips indicates that the contact stress levels in both groups were less than that needed to initiate OA development. However, the shift in maximum contact stress to later in the gait cycle for these fracture patients indicates that the surgical reductions have not returned the hip joints to a fully normal mechanical state. DEA-calculated contact stress holds potential as an early predictor of patient specific radiographic outcomes.

The influence of malalignment and ageing following sterilisation by gamma irradiation in an inert atmosphere on the wear of ultra-high-molecular-weight polyethylene in patellofemoral replacements.

Complications of patellofemoral arthroplasty often occur soon after implantation and, as well as other factors, can be due to the design of the implant or its surgical positioning. A number of studies have previously considered the wear of ultra-high-molecular-weight polyethylene patellae following suboptimal implantation; however, studies have primarily been carried out under a limited number of degrees of freedom. The aim of this study was to develop a protocol to assess the wear of patellae under a malaligned condition in a six-axis patellofemoral joint simulator. The malalignment protocol hindered the tracking of the patella centrally in the trochlear groove and imparted a constant 5° external rotation (tilt) on the patella button. Following 3 million cycles of wear simulation, this condition had no influence on the wear of ultra-high-molecular-weight polyethylene patellae aged for 4 years compared to well-positioned non-aged implants (p > 0.05). However, under the malaligned condition, ultra-high-molecular-weight polyethylene patellae aged 8–10 years after unpacking (following sterilisation by gamma irradiation in an inert atmosphere) and worn ultra-high-molecular-weight polyethylene components also aged 4 years after unpacking (following the same sterilisation process) exhibited a high rate of wear. Fatigue failure due to elevated contact stress led to delamination of the ultra-high-molecular-weight polyethylene and in some cases complete failure of the patellae. The results suggest that suboptimal tracking of the patella in the trochlear groove and tilt of the patella button could have a significant effect on the wear of ultra-high-molecular-weight polyethylene and could lead to implant failure.

Analysis of severely fractured glenoid components: clinical consequences of biomechanics, design, and materials selection on implant performance

The longevity of total shoulder replacement is primarily limited by the performance of the ultrahigh-molecular-weight polyethylene (UHMWPE) glenoid component in vivo. Variations in glenoid design (conformity, thickness), biomechanics (joint kinematics), and UHMWPE material selection (sterilization, cross-linking) distinguish total shoulder replacements from hip and knee arthroplasty devices. These variables can lead to severe mechanical failures, including gross fracture. Sixteen retrieved glenoids with severe fracture were analyzed. The explant cohort included 3 material groups (gamma-sterilized Hylamer; gamma-sterilized UHMWPE; and gas plasma-sterilized, remelted, highly cross-linked UHMWPE [HXL]) and a range of conformities (0- to 10-mm radial mismatch). Analysis included fractography (optical and scanning electron microscopy) and Fourier transform infrared spectroscopy for oxidative analysis. Fracture primarily occurred along the exterior rim for all 16 explants. Fourier transform infrared analysis and fractography revealed significant oxidative embrittlement for all gamma-sterilized glenoids. Fatigue striations and internal flaws were evident on the fracture surface of the HXL glenoid, with little oxidation detected. Fracture initiated at the external rim of all devices. Elevated oxidation levels and visible material distortion for representative gamma-sterilized conventional and Hylamer devices suggest oxidative embrittlement as a driving force for crack inception and subsequent fracture. Brittle fracture of theHXL glenoid resulted from a combination of elevated contact stress due to a nonconforming surface, an internal flaw, and reduced resistance to fatigue crack growth. This demonstrates that glenoid fracture associated with oxidation has not been eliminated with the advent of modern materials (HXL) in the shoulder domain. Basic Science Study; Implant Retrieval Study.

Comparison of ceramic‐on‐metal and metal‐on‐metal hip prostheses under adverse conditions

Ceramic-on-metal (COM) hip replacements, where the head is BIOLOX® Delta ceramic and the liner is CoCrMo alloy, have demonstrated reduced wear under standard simulator conditions compared to metal-on-metal (MOM) bearings. COM hips are now being used clinically around the world. MOM hip resurfacings have raised concerns regarding poor clinical performance and increased in vivo wear was associated with steeply inclined acetabular components and translationally malpositioned components. The aim of this study was to compare the wear rates of MOM and COM total hip prostheses under adverse edge-loading conditions in a hip simulator test. COM and MOM 36 mm hip prostheses were tested in a hip simulator, with liners mounted to provide a clinical inclination angle of 55°. A simplified gait cycle and microseparation conditions were applied for two million cycles in 25% new born calf serum. The overall mean volumetric wear rate of COM bearings under adverse conditions was 0.36 ± 0.55 mm³/million cycles; this was significantly less than MOM wear (1.32 ± 0.91 mm³/million cycles). Under these adverse conditions; the contact zone on the head intersects the rim of the cup causing substantially elevated contact stresses, disrupting the protective boundary and mixed lubrication regime causing changes in types and severity of wear mechanisms. In COM bearings, the harder head does not become damaged when there is lubricant starvation and wear does not accelerate. In conclusion, COM bearings showed reduced wear compared to MOM bearings under standard and adverse clinically relevant simulator conditions and COM bearings may provide an advantage over MOM bearings under edge-loading conditions clinically.

Femoral Head Shape Differences During Development May Identify Hips at Risk of Degeneration

Developmental dysplasia of the hip (DDH) is a common cause of elevated contact stress and early onset osteoarthritis (OA). We hypothesized that adaptation to focal loading during postnatal development would result in signature changes to the shape of the femoral head secondary center of ossification (SCO). SCO shape was evaluated in a canine model of DDH at ages 14 and 32 weeks. The evolving 3D morphology of the SCO was captured using serial quantitative computed tomography. A discrete medial representation shape model was fit to each SCO and served as the basis for quantitative thickness and bending measurements. Shape measurements were tested for associations with hip subluxation and degeneration. At 32 weeks, the SCO was thinner (flatter) in the perifoveal region, the site of focal loading; a greater bend to the SCO was present lateral to the site of thinning; SCO thinning and bending were associated with less femoral head coverage and with a higher probability of degeneration. Shape changes were not detected at 14 weeks. Measurement and visualization of SCO shape changes due to altered loading may provide a basis for identifying hips at risk of early onset OA and a tool for surgical planning of hip restructuring.

Is elevated contact stress predictive of post‐traumatic osteoarthritis for imprecisely reduced tibial plafond fractures?

Despite the widely held belief that residual incongruities from intra-articular fractures subject the joint to contact stresses that predispose to post-traumatic osteoarthritis (PTOA), objective evidence has been lacking. This study tested the hypothesis that a metric of elevated contact stress exposure would predict the onset of PTOA. The ankles of 10 tibial plafond fracture patients were treated initially using a spanning fixator, with subsequent screw fixation of the articular surface. Following up on an earlier report of finite element computed post-operative contact stress distributions in these patients' ankles, Kellgren-Lawrence (KL) scores were assessed from minimum 2-year follow-up radiographs to characterize the presence/severity of PTOA. At that time point, seven patients had developed PTOA (KL ≥ 2). Five different metrics of contact stress exposure were calculated, all of which exhibited excellent concordance with KL scores, ranging from 88% to 95%. When time of stress exposure was included, one metric was able to predict PTOA development (KL ≥ 2) with 100% reliability, and all metrics exhibited >94% prediction reliability. These findings, albeit in a small population, support the existence of a contact stress exposure threshold above which incongruously reduced tibial plafond fractures are highly likely to develop PTOA.

Femoral Head Bone Mineral Density Patterns May Identify Hips at Risk of Degeneration

Developmental dysplasia of the hip (DDH) is a common abnormality that causes elevated contact stress and early onset osteoarthritis (OA). We hypothesized that adaptation of femoral head bone mineral density (BMD) patterns to focal loading during postnatal development could be used to identify hips at risk of degeneration. Evolving BMD patterns of the femoral head secondary center of ossification (SCO) were quantified and tested for differences with hip subluxation and degeneration. BMD was measured using quantitative computed tomography of hips in a canine model of DDH from 4weeks to early skeletal maturity at 32weeks. During body weight and SCO volumetric growth deceleration, SCO mean BMD increased rapidly and local regions of high BMD formed. Greater subluxation was associated with a lower mean BMD up to 14weeks. At 32weeks, greater subluxation was associated with a larger area of high BMD that was more laterally located and had a greater maximum BMD. BMD differences were associated with a higher probability of cartilage degeneration. Measurement and visualization of BMD pattern changes due to altered mechanical loading provide a basis for identifying hips at risk of early onset OA and a tool for surgical planning of contact stress reduction procedures.

Influence of the meniscus on friction and degradation of cartilage in the natural knee joint

Total meniscectomy has been shown to cause early-onset arthritis in the underlying cartilage and bone in the knee joint, demonstrating that the meniscus plays an important protective role in the load carrying capacity. Relationships between friction and wear in synovial joints are complex due to the biphasic nature of articular cartilage and the time dependency of tribological responses. Determination of friction and wear in the whole natural joint in vitro or in vivo is technically difficult and the tribological effect of meniscectomy has not been previously studied in an articulating knee joint. The aim of this study was to use a tribological simulation of the medial compartmental bovine knee, to investigate friction and wear, with and without the meniscus. We hypothesised that meniscectomy would lead to elevated contact stress and frictional coefficient across the joint. Skeletally mature bovine medial compartmental knee joints were dissected and mounted in a pendulum friction simulator, which was used to apply physiologically relevant loading and motion. Wear was quantified using micro-MRI scans and surface profilometry. Knees tested with the intact meniscus showed no change in surface roughness and no detectable cartilage loss or deformation. However, increased contact stress and frictional coefficient upon removal of the meniscus, led to immediate surface fibrillation, biomechanical wear and permanent deformation of cartilage. This study presents, for the first time, an in vitro model simulation system to investigate the tribological effects of meniscectomy and meniscus repair and regeneration.

ＭＲＩを用いた関節の接触領域の測定

Elevated contact stress on the articular joints has been hypothesized to contribute to articular cartilage wear and joint pain. However, given the limitations of using contact stress and areas from human cadaver specimens to estimate articular joint stress, there is need for an in vivo method to obtain such data. Magnetic resonance imaging (MRI) has been shown to be a valid method of quantifying the human joint contact area, indicating the potential for in vivo assessment. The purpose of this study was to describe a method of quantifying the tibiofemoral joint contact area using MRI. The validity of this technique was established in porcine cadaver specimens by comparing the contact area obtained from MRI with the contact area obtained using pressure-sensitive film (PSF). In particular, we assessed the actual condition of contact by using the ratio of signal intensity of MR images of cartilage surfaces. Two fresh porcine cadaver knees were used. A custom loading apparatus was designed to apply a compressive load to the tibiofemoral joint. We measured the contact area by using MRI and PSF methods. When the ratio of signal intensity of the cartilage surface was 0.9, the error of the contact area between the MR image and PSF was about 6%. These results suggest that this MRI method may be a valuable tool in quantifying joint contact area in vivo.

Shear Stress in Epiphyseal Growth Plate is a Risk Factor for Slipped Capital Femoral Epiphysis

Slipping of the capital femoral epiphysis is an important orthopaedic problem of early adolescence. Many hypotheses about its etiology have been examined, yet the underlying mechanisms have not yet been fully elucidated. We examined elevated shear stress in the epiphyseal growth plate and elevated contact hip stress exerted on the femoral head as risk factors for slipping of the capital femoral epiphysis. Two groups of hips were compared: a group of 100 hips contralateral to the slipped ones and a group of 70 age- and gender-matched healthy hips. The characteristics of individual hips were incorporated by means of geometrical parameters determined from standard anteroposterior radiographs. Shear stress was calculated by using a mathematical model where the femoral neck was considered to function as an elastic rod. Contact hip stress was calculated by the HIPSTRESS method. Hips contralateral to the slipped ones had higher average shear stress (0.81 vs 0.51 MPa; P < 0.001) and more vertically inclined physeal angle (55.4 vs 63.2 degrees.; P < 0.001) in comparison to healthy hips. Shear stress in the contralateral hips to the slipped ones remained significantly higher even when normalized to the body weight (1400 vs 1060 Pa/N; P < 0.001). There was no significant difference in the average contact hip stress (1.86 vs 1.74 MPa; P = 0.145). Elevated shear stress, but not elevated contact stress, is a risk factor for slipping of the capital femoral epiphysis. III (prognostic study, case-control study).

Contact Stress in Hips with Osteonecrosis of the Femoral Head

Contact stress distribution in the articular surface of the hip is considered a factor in the development of osteoarthritis, a common complication in hips with aseptic necrosis of the femoral head. We present evidence supporting the hypothesis that osteoarthritis in hips with aseptic necrosis of the femoral head can be caused by elevated contact stress related to the reduced load-bearing ability of the necrotic bone. By using a previously validated mathematical model, we observed that hip contact stress may increase considerably if the load-bearing capacity of the necrotic lesion is decreased, if the size of the necrotic segment is increased, and if the necrotic segment is located more laterally. These effects are affected by the intrinsic shape of the hip. As the estimated values of stress in hips with osteonecrosis are in the range obtained by the same method in dysplastic hips, osteoarthritis in hips with osteonecrosis can be caused by elevated contact stress.

Comparison of Capacitive versus Resistive Joint Contact Stress Sensors

Cartilage contact stress elevations might be associated with pain or other symptoms after malunited, incongruous intraarticular fractures. Studies identifying fractures with patterns of elevated contact stresses would help to ensure more appropriate choices of treatment. However, appropriate instrumentation for such studies is crucial. We tested two such systems, one capacitive and one resistive, under identical loading conditions presumed to occur in the ankle. We used a materials testing machine and customized-loading fixtures to measure force detection error, contact area error, repeatability, homogeneity, creep, and one-axis and two-axis bending artifacts. The loading regimen caused pressures up to 2.5 MPa. An error in force detection between -3% and +5% was observed with the capacitive sensor whereas an error between -12% and +20% was observed with the resistive sensor. Repeatability and homogeneity were greater for the capacitive sensor. Errors in contact area measurement were less than 2% for the resistive sensor and less than 6% for the capacitive sensor. The resistive sensor could not conform to spherical surfaces without crinkling. Creep artifact was observed with both sensors. We concluded that the capacitive sensor had superior performance even though its thickness and high compliance may be disadvantageous in intraarticular measurements. The resistive sensor is required for use where higher pressures are expected despite its inferior accuracy.

Importance of pin geometry on pin-on-plate wear testing of hard-on-hard bearing materials for artificial hip joints.

The contact mechanics between the pin and the plate used in simple wear screening tests were investigated in this study. Both soft-on-hard, such as ultra-high molecular weight polyethylene (UHMWPE)-on-metal or UHMWPE-on-ceramic, and hard-on-hard, such as metal-on-metal, bearing couples were considered. The effect of the pin geometry and the misalignment between the pin and the plate were investigated on the predicted contact pressure distribution at the bearing surfaces using the finite element method. It was demonstrated that in the case of soft-on-hard bearing couples, neither the geometrical discontinuity of the pin surface nor the misalignment could cause a significant increase in the contact stress. However, for hard-on-hard combinations, even with a very small misalignment of 0.5 degrees between the pin and the plate, the geometrical discontinuity could lead to a more than tenfold increase in the predicted contact stress. This elevated contact stress may lead to a large scatter in the wear data and, even more importantly, structural damage of the bearing surfaces.

Effect of osteochondral defects on articular cartilage. Contact pressures studied in dog knees.

Full thickness osteochondral defects 6 mm in diameter were created in the weight-bearing regions of the femoral condyles in 5 adult mongrel dogs to study the contact pressure changes accompanying healing. A digital imaging technique employing Fuji Prescale film mapped contact pressures following 11 months of healing for comparison with contralateral normal knees, and knees with freshly made defects. Although all defects healed uneventfully with subchondral plate reconstitution and with growth restoration of the articular surface, the repair soft tissue appeared histologically to be primarily fibrous tissue, with varying degrees of a fibrocartilaginous component. The mean and peak stresses about fresh defects were not appreciably different from those about healed defects. Neither were there substantial differences in the total cartilage area making contact, except when very low loads were applied. The results suggest that the repair tissue is of poor mechanical quality, and does not contribute appreciably to weight bearing. The cartilage adjacent to the defect did not experience high stresses; neither gross nor light microscopic evidence of degeneration appeared at 11 months. If degeneration does occur following such defects, our data suggest that it is not because of elevated contact stresses.