Frictional resistance to tendon gliding is minimised by surrounding loose areolar tissues. During periods of prolonged immobilisation, for example post tendon-repair, adhesions can form between the two adjacent tissues, thereby limiting function. Whilst agents applied during surgery are recognised to succeed in adhesion prevention, they have also been reported to provide some reduction in friction during in vitro tendon-bony pulley investigations. This study investigated the effectiveness of common anti-adhesion agents in lubricating the tendon-surrounding tissue contact by comparison with a control study. By using a validated apparatus and with reference to the Stribeck curve, it was determined that the natural in vivo contact is likely to be lubricated by a film of synovial-like fluid. Application of all anti-adhesives generated a similarly efficient lubricating system, and hence administration of these agents should be encouraged to all regions of the tendon disrupted during surgery. Minimising frictional resistance to gliding will reduce the likelihood of tendon ‘gapping’ - and subsequently failure - at the repair site.

Paratenonitis describes inflammation of the paratenon and commonly presents as an overuse injury. The paratenon is the connective tissue sheath that surrounds tendons - including tendo Achilles, and serves to minimise friction with the outer layer of the tendon, the epitenon. Whilst this conjunction allows the tendon to glide smoothly on muscular contraction, the presentation of paratenonitis typically follows periods of frequent, repetitive musculoskeletal movements; hence, paratenonitis commonly afflicts the elite and, albeit to a lesser extent, amateur athlete. The extent to which friction at the epitenon-paratenon juncture contributes to this tendinopathy remains unclear, and this study is therefore concerned with the coefficient of friction and the lubrication regime.

By using a specially designed and validated apparatus, the in vivo paratenon-epitenon conjunction was approximated using bovine flexor tendon paratenon and a glass disc; this is being an equivalent experimental set-up to that used in other studies exploring soft tissue contacts. Bovine synovial fluid was used to lubricate the conjunction at 37 deg C, and the frictional characteristics were analysed over a range of sliding speeds and loads.

The coefficient of friction was found to generally lie between 0.1 – 0.01. This range suggests that a system of mixed lubrication applies - where the synovial fluid is causing partial separation of the two surfaces. However, when the data is plotted in the form of a Stribeck curve, the trend suggests that boundary lubrication prevails - where lubrication is determined by surface-bound proteins.

The coefficient of friction at the epitenon-paratenon interface appears to be approximately one order of magnitude greater than that typically reported within the healthy synovial joint. Additionally, the synovial joint is thought to exhibit some fluid film lubrication (i.e. total surface separation), whereas the epitenon-paratenon lubrication regime appears to vary only between the inferior mixed and boundary systems - depending on the specific biomechanical conditions. This data would suggest that the coefficient of friction at the epitenon-paratenon interface is relatively high and thus is potentially significant in the incidence of paratenonitis. Such a hypothesis could be of particular interest to sports-medicine and orthopaedic specialists.

Purpose of study There is renewed scientific interest in the use of metal-metal bearings for hip replacements. Such bearings have lower volumetric wear rates compared to metal or ceramic on polyethylene bearings. They permit the use of large diameter bearings which potentially have the benefit of reduced dislocation. They also allow the use of thin components without the risk of fracture associated with similar ceramic-ceramic components. However, there remain concerns about the long-term effects of nanometre sized debris and the release of metal ions. It is therefore critical to understand which parameters are important in minimising the amount of debris generated. This study investigated the effect of design and materials on the wear rates in a hip simulator.

Methods Wear studies were carried out in a 10 station ProSim hip simulator in 25% newborn calf serum. A Paul type load curve was applied (maximum load 3000N, minimum 300N) in an anatomical configuration. The extent of a fluid film between the bearing surfaces was determind by measuring the voltage drop between the components. Test samples were made from low-carbon (< 0.05%) and high-carbon (> 0.20%) CoCrMo alloys in various conditions. These samples had bearing surface diameters of 16–54.5mm. The diametral clearance between the femoral head and acetabular cups were from 50–300um.

Results The results of this study were that the low-carbon material wears more than high-carbon materials, there is no significant difference in wear performance of the various forms of high-carbon material tested (wrought, cast, and cast and heat treated), and wear decreased with reduced clearances and increased component diameter. Voltage changes indicated that reduced clearances resulted in component separation and fluid film lubrication

Conclusions These results are consistent with the hypothesis that large diameter metal-on-metal bearings with optimized bearing surface geometry operate in the mixed and/or fluid film lubrication regime.