We studied the mechanical properties of cartilage from the apparently unaffected compartment of knees with unicompartmental osteoarthritis (OA). Plugs of cartilage and subchondral bone, 8 mm in diameter, were obtained from the tibial plateau of seven patients treated by total knee replacement. Control specimens were obtained from eight cadaver knees of similar age. Specimens were loaded by a plane-ended indentor in a hydraulic materials testing machine; we measured thickness, 'softness', rate of creep, and compressive strength of the articular cartilage. We found that the 'unaffected' cartilage from OA knees was significantly thinner and softer than control cartilage, and it was slightly, although not significantly, weaker. We conclude that the apparently unaffected cartilage in knees with unicompartmental OA is mechanically inferior to normal cartilage, even although clinically, radiologically and morphologically it appears to be sound.

Diaphyseal fractures of the tibia in 80 patients were treated by external skeletal fixation using a unilateral frame, either in a fixed mode or in a mode which allowed the application of a small amount of predominantly axial micromovement. Patients were allocated to each regime by random selection. Fracture healing was assessed clinically, radiologically and by measurement of the mechanical stiffness of the fracture. Both clinical and mechanical healing were enhanced in the group subjected to micromovement, compared to those treated with frames in a fixed mode possessing an overall stiffness similar to that of others in common clinical use. The differences in healing time were statistically significant and independently related to the treatment method. There was no difference in complication rates between treatment groups.

Diurnal changes in the loads acting on the spine affect the water content and height of the intervertebral discs. We have reviewed the effects of these changes on spinal mechanics, and their possible clinical significance. Cadaveric lumbar spines subjected to periods of creep loading show a disc height change similar to the physiological change. As a result intervertebral discs bulge more, become stiffer in compression and more flexible in bending. Disc tissue becomes more elastic as its water content falls, and its affinity for water increases. Disc prolapse becomes more difficult. The neural arch and associated ligaments resist an increasing proportion of the compressive and bending stresses acting on the spine. Observations on living people show that these changes are not fully compensated for by modified muscle activity. We conclude that different spinal structures are more heavily loaded at different times of the day. Therefore, the time of onset of symptoms and signs, and any diurnal variation in their severity, may help us understand more about the pathophysiology of low back pain and sciatica.

Cadaveric lumbar discs were injected with chymopapain and subjected to a series of mechanical tests over a period of up to 19 hours. Discs from the same spine injected with saline were used as controls. The results showed that chymopapain had no measurable effect on the mechanical properties of the disc apart from the increased height and stiffening caused by fluid injection. Another series of tests on isolated pieces of disc material showed that chymopapain could reduce the size of prolapsed nuclear material by 24% in one hour and by 80% in 48 hours. It is concluded that, in the short-term, chymopapain has a negligible effect on the mechanics of a disc but it can reduce the size of any prolapsed nuclear material with which it comes in contact.

One hundred and thirty-nine discs from cadaveric lumbar spines were injected with a mixture of radio-opaque fluid and dye. Discograms were taken and the discs were then sectioned in the sagittal plane. Examination of the sections revealed that injected fluid did not at first mix with the disc matrix but pushed it aside to form pools of injected fluid. The location of these pools, and hence the appearance of a discogram, depended on the stage of degeneration of the disc. It is concluded that useful clinical information can be obtained from discograms.

A series of experiments showing how posture affects the lumbar spine is reviewed. Postures which flatten (that is, flex) the lumbar spine are compared with those that preserve the lumbar lordosis. Our review shows that flexed postures have several advantages: flexion improves the transport of metabolites in the intervertebral discs, reduces the stresses on the apophyseal joints and on the posterior half of the annulus fibrosus, and gives the spine a high compressive strength. Flexion also has disadvantages: it increases the stress on the anterior annulus and increases the hydrostatic pressure in the nucleus pulposus at low load levels. The disadvantages are not of much significance and we conclude that it is mechanically and nutritionally advantageous to flatten the lumbar spine when sitting and when lifting heavy weights.

Cadaveric lumbar spine specimens of "motion segments", each including two vertebrae and the linking disc and facet joints, were compressed. The pressure across the facet joints was measured using interposed pressure-recording paper. This was repeated for 12 pairs of facet joints at four angles of posture and with three different disc heights. The results were that pressure between the facets increased significantly with narrowing of the disc space and with increasing angles of extension. Extra-articular impingement was found to be caused, or worsened, by disc space narrowing. Increased pressure or impingement may be a source of pain in patients with reduced disc spaces.

Forty-one cadaveric lumbar intervertebral joints from 18 spines were flexed and fatigue loaded to simulate a vigorous day's activity. The joints were then bisected and the discs examined. Twenty-three out of 41 of the discs showed distortions in the lamellae of the annulus fibrosus and, in a few of these, complete radial fissures were found in the posterior annulus.

Cadaveric lumbar intervertebral joints were loaded to simulate the erect standing posture (lordosis), and the erect sitting posture (slightly flexed). The results show that, after the intervertebral disc has been reduced in height by a period of sustained loading, the apophysial joints resist about 16 per cent of the intervertebral compressive forces in the erect standing posture, whereas in the erect sitting posture they resist none. The implications of this in relationship to degenerative changes and to low backache are discussed.