Subchondral bone plate thickening precedes chondrocyte apoptosis and cartilage degradation in spontaneous animal models of osteoarthritis.

Osteoarthritis (OA) is the most common joint disorder characterised by bone remodelling and cartilage degradation and associated with chondrocyte apoptosis. These processes were investigated at 10, 16, 24, and 30 weeks in Dunkin Hartley (DH) and Bristol Strain 2 (BS2) guinea pigs that develop OA spontaneously. Both strains had a more pronounced chondrocyte apoptosis, cartilage degradation, and subchondral bone changes in the medial than the lateral side of the tibia, and between strains, the changes were always greater and faster in DH than BS2. In the medial side, a significant increase of chondrocyte apoptosis and cartilage degradation was observed in DH between 24 and 30 weeks of age preceded by a progressive thickening and stiffening of subchondral bone plate (Sbp). The Sbp thickness consistently increased over the 30-week study period but the bone mineral density (BMD) of the Sbp gradually decreased after 16 weeks. The absence of these changes in the medial side of BS2 may indicate that the Sbp of DH was undergoing remodelling. Chondrocyte apoptosis was largely confined to the deep zone of articular cartilage and correlated with thickness of the subchondral bone plate suggesting that cartilage degradation and chondrocyte apoptosis may be a consequence of continuous bone remodelling during the development of OA in these animal models of OA.

Biomechanical implications of degenerative joint disease in the apophyseal joints of human thoracic and lumbar vertebrae.

An experimental technique for quantifying load-sharing in cadaveric spines is used to test the hypothesis that degenerative changes in human apophyseal joints are directly related to high levels of compressive load-bearing by these joints. About 36 cadaveric thoraco-lumbar motion segments aged 64-92 years were subjected to a compressive load of 1.5 kN. The distribution of compressive stress was measured in the intervertebral discs using a miniature pressure transducer, and stress measurements were summed over area to give the compressive force resisted by the disc. This was subtracted from the applied 1.5 kN to indicate compressive load-bearing by the apophyseal joints. The cartilage of each apophyseal joint surface was then graded for degree of degeneration. After maceration, each joint surface was scored for degenerative joint disease (DJD) affecting the bone. Results demonstrated that the apophyseal joints resisted 5-96% (mean 45%) of the applied compressive force. A significant positive correlation was demonstrated between age and cartilage degeneration, age and DJD bone score, apophyseal joint load-bearing and bone score, and cartilage score and load-bearing. The latter correlation was strongest for load-bearing above 50%. Ordinal regression showed that the variables describing bone DJD (marginal osteophytes, pitting, bony contour change, and eburnation) were significantly correlated with degree of cartilage degeneration. It is concluded that in elderly individuals apophyseal joint load-bearing above a threshold of 50% is associated with severe degenerative changes in cartilage and bone, and that markers of DJD observed palaeopathologically may be used as predictors of such loadingin life.

Internal stress distribution in cervical intervertebral discs: the influence of an artificial cervical joint and simulated anterior interbody fusion.

There is concern that cervical interbody fusion can result in accelerated degenerative changes occurring at adjacent spinal levels. The cervical spine clearly evolved to be mobile. It would seem to be desirable for spinal surgeons to have an alternative to fusion, and spinal arthroplasty is an appealing concept. The Bristol Disc is a mechanical device comprising two articulating components that result in motion with 6 df. It has been shown to have favorable kinematics when compared with intact and fused cadaveric spines. The current study attempts to record changes in the distribution of stresses within cervical intervertebral discs adjacent to the artificial disc or a simulated fusion. The technique used to measure intradiscal stress distributions is based on earlier work by McNally and Adams on lumbar intervertebral discs. The study generated stress profiles through cervical intervertebral discs statically loaded in four different postures in addition to recording changes in intradiscal pressure within both the nucleus and the annulus during flexion. Similar stress profiles were recorded from intact specimens and those with the artificial joint inserted. The artificial joint resulted in reduced stresses in the annulus compared with spines with a simulated fusion. The study demonstrates how different testing conditions can result in researchers being confronted with paradoxical data, and the simulation of muscle forces is recommended.