The objective of this study was to determine the tensile strength of the different components of rotator cuff tendons.

A test bench that performs tests at consistent rupture speed was used to do separate tensile tests on 10-mm strips of capsular and tendinous layers in four fresh frozen cadaveric shoulders. The layers were left attached only on the humeral side.

The maximum force was comparable but the elongation of the outer part of the tendon was greater, indicating that the capsular part would tear first. On average, a 10-mm strip of capsular layer failed at 170N with elongation of 7 mm, while a 10-mm strip of tendinous layer failed at 230N with elongation of 10 mm.

Using six fresh frozen cadaveric shoulders, we went on to determine the strength of the rotator hood, a thin layer of tendon extending beyond the tuberosity major and connecting the supraspinatus to the subscapularis via the bicipital tunnel. The rotator hood ruptured at a mean force of 70 N.

We concluded that the two layers of the cuff contribute equally to the strength. It is therefore important to repair both layers. The difference in elongation of the tendinous and capsular layers makes the capsular layer more vulnerable to elongation stress. The rotator hood is a strong and important structure, and it is important to repair it.

We conducted an engineering failure analysis of retrieved acetabular cups. From one centre, 37 properly-marked components were retrieved. The details of the patients were noted. Of the 37 components, 27 were brought to the laboratory for an engineering investigation of the cause of failure. A further 10 components were also taken to the biochemistry laboratory within an hour of retrieval, with tissue removed from the patients.

The purpose of the investigation was to determine whether any proteins were deposited inside the cups and to see whether there was wear debris on either the retrieved component or in the tissue surrounding the prosthesis. We used visual inspection, colour-dye penetrant, stereo-microscopes, scanning electron microscopes, and mass-spectometric analysis to examine the cups. Debris was captured using 0.4-um filters.

We found mechanical failure in vivo is mainly caused by plastic flow. Fretting is the second most likely cause of failure. Both of these are indicative of localised overheating between the acetabular component and the ceramic femoral head. The most likely cause of the overheating is lack of lubrication. With electrophoresis it became evident that at some time in their in-vivo service these cups had reached temperatures exceeding 60°C. These temperatures were confirmed on a five-poster hip simulator.

We suggest that an in-depth study be undertaken to establish the method of in-vivo lubrication and the lubricity of the available lubricant.

The mechanical failure of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups in vivo is due mainly to a combination of excessive plastic flow and fretting. Localised overheating of the bearing surface, due to insufficient lubrication, causes this. The purpose of this study was to determine the amount of creep in UHMWPE under various conditions.

Test pieces were cut from a piece of raw material and tested according to ASTM D2990. In the first test, to determine the anisotropic behaviour of the material, test pieces of raw material were cut at various orientations. The material was then tested in the virgin state and the virgin state at different temperatures. It was also gamma sterilised under different conditions, namely 24 kGy in air, 25 kGy in a nitrogen atmosphere and 25 kGy in air, and heat treated at 80°C to get an annealing effect. Further tests were conducted to determine the effect of cross-linking on creep behaviour. These tests were administered at room temperature, at 50°C and at 60°C.

The material showed extreme anisotropic behaviour. It was more sensitive to creep in the centre of the bar than on the outside (32%). Maximum creep, however, occurred at a 45°-angle. This is significant if we assume that maximum loading of an acetabular cup occurs at an angle of 70.7°. The difference in creep for the virgin material, measured at room temperature and at 60°C, was 87.3% or 0.716 mm. The variance in creep for the different methods of sterilisation was a maximum of 0.3 mm. Creep for the cross-linked material, however, was markedly less than for the virgin material. There was a decrease of 36% (0.58 mm) in creep at room temperature and almost 83% (0.84 mm) at 60°C.

The test results show that the cross-linked material is much more stable. This may explain the good in-vivo service of these products.

Information acquired from retrieved polyethylene ace-tabular components is invaluable to the design engineer in achieving better in-vivo service. The failure criteria used, namely mode-1 to 4 wear, are vague and not user-friendly. Based on an in-depth evaluation of over 100 failed acetabular cups, this study investigated a more precise classification

Although all the cups were obtained from one centre, they were not clearly marked, making an accurate assessment of the in-vivo life impossible. This, however, was not the aim of the study. We used visual inspections, magnifying glasses, colour-dye penetrant and stereo-microscopes to examine the cups. The most common defects identified were mechanical damage, cracks in the material, plastic flow, scratches, fretting, flaking and wear particles embedded in base material. These cups provided valuable data for compilation of a proposed set of failure criteria to be used in future. Visible defects should be used as a classification tool in future cup-failure analysis. They are explicit and can be used with confidence.

In high-demand situations, modern thinking and experience in total hip arthroplasty (THA) favours the uncemented press-fit cup over its cemented counterpart. Before its regular use in 1996, a high-demand cemented stem was designed for use as a short revision stem with a press-fit cup, with or without impaction bone grafting, in active people, especially those over 55 years.

Conceptually, a collarless double-tapered highly polished design was preferred. The clip-on hollow centraliser was designed for 5-mm subsidence. The valgus stem, with cement superior to the shoulder, limited upward pistoning in the cement sleeve, creating less debris. The stiff upper and flexible distal part resulted in a decreased contribution from shear and an increased contribution from compression in load transfer from prosthesis to cement. Three sizes are available: G1, G2 and G3. A straight type is presently being developed for smaller patients with congenital dysplasia of the hip.

The stem, made by (Thornton Heavy Engineering Sheffield, United Kingdom), from Rex 734 stainless steel to ISO-2002 standards, tapers 10 mm to 12 mm (6°). All tolerances are adequate to handle Inox or Ceramic heads.

From April 1996 to December 2002, 278 stems were implanted in Dr Weber’s practice. The first 172 hip operations (168 patients) were studied. The mean age was 58.6 years. There were 137 primary hips and 25 revisions.

The mean follow-up period was 4.5 years (3 to 7). Three patients died with the prosthesis in situ. Two reoperations were done: one cup was revised for recurrent dislocation and one fracture below the step was successfully plated. Only three cases of subsidence were documented, all of them less than 3 mm. To date there have been no stem revisions.

The prosthesis, together with the stainless steel head and cross-linked cup, can be regarded as cost-effective and can be used routinely, as a high-demand prosthesis with press-fit cup, or as a short-revision prosthesis.