The ankle joint is a stable and congruent joint that helps to protect the joint surfaces from high impact forces. However, possible trauma to the joint such as severe ankle sprain or fracture can cause cartilage breakdown and eventually lead to cartilage degeneration, resulting in arthritis. Ankle arthritis is considered to be a major cause of morbidity and disability. Although the ankle joint is least affected by arthritis compared to knee and hip joints, the pain and lack of mobility of end stage ankle osteoarthritis (OA) are equally debilitating and tend to be overlooked compared to hip and knee OA. Differences in the incidence rates of osteoarthritis (OA) across the joints could be partly attributed to the biomechanical properties of the articular cartilage.
The aim of the thesis was to improve the understanding of mechanical characteristics of the human ankle cartilage through developing and refining methodologies (i.e. indentation and thickness methods) on immature porcine ankle tissues. As porcine ankle joint seems to closely represent the human ankle with comparable anatomical features, cartilage deformation, cartilage thickness, coefficient of friction, surface roughness, contact mechanics and biological properties were also determined. Comparisons of mechanical characteristics between porcine and human tissues were reported.
A methodology was developed to identify the most suitable type of specimen (osteochondral samples versus whole joints) for mechanical characterisation as specimen preparation via pin extraction was hypothesised to have an effect on the tissue quality and thus on biomechanical properties. Specimen preparation of osteochondral pins had no impact on properties as cartilage deformation and thickness measurements of pins were comparable to whole joints. Therefore, for mechanical characterisation of human ankle cartilage, osteochondral pins were studied.

Porcine talar cartilage was found to be thicker, with higher surface roughness, increased water content, increased contact pressures and lower glycosaminoglycan (GAG) content compared to porcine tibial cartilage. Based on such results, the talar cartilage in the young porcine tissue (3 to 6 months) appeared more susceptible to deterioration over time when compared to tibial cartilage as these properties were considered as unfavourable potentially affecting joint function and quality of tissue during high impact forces. Overall, there were significant differences in thickness, deformation and roughness measurements (ANOVA, p<0.05 for all comparators) across the porcine and human tissues. These differences between animal and human tissues can be attributed to many factors such as age, gait, lifestyle and mechanical properties. The immature porcine cartilage was considered to be a poor representative model for tribological studies.
On the human ankle joint, cartilage thicknesses, deformation and surface roughness measurements were all in a comparable range between talar and tibial joint surfaces (ANOVA, p<0.05 for all comparators). Although ankle lesions were reported to be commonly found in the talar surface rather than the tibial surface, and it was assumed to result in unfavourable properties, this was not reported in the current study as no significance was observed between both joint surfaces.

Biotribology of the natural ankle joint

Clinical reports of surgical intervention options, such as spacers or hemi-arthroplasties, particularly for treatment of young arthritic patients, have been poor [[1][1]]. Knowledge of the tribology of the cartilage-prosthesis interaction of these devices would potentially provide an insight to the reasons for the premature failure of these devices and the development of more appropriate intervention treatment solutions for arthritic patients. Frictional studies of articular cartilage have been reported, using simple pin-on-plate geometric configurations [[2][2]], which do not accurately represent the geometric and stress conditions in the natural joint. A more representative model, based on the medial compartment of the knee joint has been developed in the Part 1 of this study [[3][3]] for the pre-clinical tribological testing the natural joint and their related arthroplasty devices. Bearing geometry is an important consideration in limiting wear, as shown in congruous meniscal knee replacement, which exhibited lower wear rates than incongruous designs [[4][4],[5][5]]. The aim of this study was to use a unicompartmental hemi-arthroplasty model to examine the effect of tibial conformity and stress on the friction and wear of articular cartilage.

Experiments were conducted in an anatomic pendulum friction simulator (SimSol, UK) using the medial femoral condyle of a bovine knee joint articulating against two conforming stainless steel (316L) tibial plates (R=50mm and 100mm). A simplified physiologic knee loading profile was applied represent both low loading and much higher physiological loading conditions, with peak load between 259N – 1.5kN). Tests were conducted in 25% bovine serum and run for 3600 and 300 cycles under the low and high loading conditions respectively. The motion was cycled at 1Hz with amplitude between −10°–13.1°. Cartilage wear was assessed qualitatively from surface roughness measurements using a surface profile using a surface profilometer (Taylor Hobson, UK). The friction and wear of cartilage articulating against the conforming tibial plates were compared to a positive control flat tibial plate model [[3][3]]. The conforming plate models were found to produce significantly lower cartilage friction and surface damage (μ=0.022–0.035, Ra=0.136–0.145μm) than the flat plate model (μ=0.078, Ra=2.70μm). No damage on the cartilage surface was observed under low loads, however, under higher, more physiological loading cartilage friction increased (μ=0.08) in the conforming plate model, with a significant surface damage. An anatomic unicompartmental knee joint model has been developed to successfully examine the effect of counterface conformity on cartilage friction and wear for pre-clinical testing of a hemi-arthroplasty device. Counterface conformity was shown to significantly reduce cartilage friction and wear. This was attributed to the increased surface area and reduced stresses experienced in comparison to an incongruent bearing articulation.

 [1]: #ref-1
 [2]: #ref-2
 [3]: #ref-3
 [4]: #ref-4
 [5]: #ref-5

Friction and wear testing of articular cartilage in the medial compartment of the knee using an anatomic simulator: part 2-the effect of tibial plate conformity

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Friction and wear testing of articular cartilage in the medial compartment of the knee using an anatomic simulator: part 2-the effect of tibial plate conformity