Osteolsyis is one of the main reasons for revision of total joint replacements. The osteolytic reaction is influenced by dose, size (particles < 1μm are believed to be biologically more active) and shape of wear particles, so that low wear rates and biologically less active particles are required.

In addition, in the knee a range of design and kinematic variables have to be considered as they can markedly influence wear regardless of the type of polyethylene used. Furthermore, UHMWPE (ultra-high-molecular-weight-polyethylene) fatigue occurs more frequently in the knee joint than in the hip joint due to its changing tribocontact areas combined with high weight bearing. This is why crosslinked polyethylene (XPE) is still controversially discussed for use in total knee prostheses.

Question: Is XPE appropriate for both fixed- and mobile-bearing knee prostheses? Are XPE-particles different from UHMWPE-particles?

Method: In a knee-joint-simulator (Stallforth-Ungeth-uem) 4 XPE (1 sequential irradiated/annealed, 3 different remelted, fixed- and mobile-bearing-types) and 2 UHMWPE-inserts (fixed- and mobile-bearing) were tested (ISO). The gravimetric wear rates (mg/year) were measured (5mill. cycles), the wear mechanism was analysed by scanning electron microscope (SEM). Furthermore, 100.000 particles were analysed by SEM (20nm-nucleo-pore-filter;acid digestion method;ISO) in size and shape.

Results: All the inserts showed traces of abrasion, scratching and wear polishing. XPEs produced lower wear rates (range 0,6–4,3mg/year;p< 0,05) than UHM-WPEs (range 8,4–8,5mg/year) in fixed- as well as mobile-bearing knee prostheses without fatigue reactions. The sequentially irradiated and annealed insert showed the lowest wear rate (0,6mg/year;p< 0,05) overall. For all the groups the particles were smooth, granular, irregular and less fibrillar and more than 85% of the particles were submicron. The particle size was independent of the radiation dose. Fixed-bearings produced a larger amount of particles < 1μm (93,2–96,3%;p< 0,05) than mobile-bearings (85,5–89,5%).

Conclusion: All crosslinked tibial inserts, fixed- as well as mobile-bearings, showed statistically significant (p< 0.05) reduced wear rates without any fatigue reactions. The fixed-bearing sequential irradiated and annealed insert had the lowest wear rate (p< 0.05). XPEs (fixed- and mobile-bearings) and UHMWPEs have similar wear particles in shape. The particle size was independent of the radiation dose, but most of the particles are in the biologically more active range of size (fixed> mobile-bearing). Due to this further investigation with in-vivo-models is required.

Periprosthetic osteolysis, caused in a chronic inflammatory adverse reaction to wear particles in the surrounding tissues, is one of the major reasons for revision arthroplasty so that articulating surfaces with low wear rates are required. Compared with conventional ultra high molecular weight polyethylene (UHMWPE), highly crosslinked polyethylene (HXLPE) shows a reduced wear rate in a hip simulator. The crosslinking process which is achieved by gamma or electronic radiation, followed by heat treatment either above the melting point (remelting) or below (annealing), reduces the mechanical properties of UHMWPE, particularly its fatigue strength. UHMWPE fatigue occurs more frequently in the knee than in the hip due to its higher contact stresses. This is why HXLPE is still controversially discussed for use in total knee prostheses. We have examined the wear behaviour of different HXLPEs [one cruciate-retaining (CR; sequential irradiation and annealing), one ultra-congruent (remelting), one CR (remelting)], compared with conventional UHMWPE in a knee simulator (Stallforth-Ungethuem). In the fixed bearing knee recommended from the manufacturer the wear rates [gravimetric (mg/year); volumetric (mm³/year)] were determined according to the ISO standard and the wear mechanism was analysed by means of a scanning electron microscope.

All insert showed signs of abrasion, scratching and wear polishing, but no traces of fatigue reactions. All HXLPEs produced lower (p< 0.05) wear rates (0.47–3.3 mg/year; 0.5–3.5 mm3/year) than the UHMWPE (8.1–9.1 mg/year; 8.6–9.7 mm3/year), the inserts of HXLPE manufactured by sequential irradiation and annealing showed the lowest wear rates (p< 0.05) overall.

Due to the reduced wear rates without any fatigue symptoms, we conclude that HXLPE is suitable for total knee prostheses and a monitored clinical investigation can be recommended. HXLPE manufactured by sequential irradiation and annealing seems to produce still lower wear rates than those manufactured by remelting, at least when used in total knee prostheses.

Our aim was to examine the potential of autologous perichondral tissue to form a meniscal replacement. In 18 mature sheep we performed a complete medial meniscectomy. The animals were then divided into two groups: 12 had a meniscal replacement using strips of autologous perichondral tissue explanted from the lower rib (group G) and six (group C) served as a control group without a meniscal replacement. In all animals restriction from weight-bearing was achieved by means of transection and partial resection of tendo Achillis. Six animals (four from group G and two from group C) were each killed at 3, 6 and 12 months. The grafts and the underlying articular cartilage were removed and studied by gross macroscopic examination, light microscopy, SEM, polarised light examination, and by biomechanical tests.

In all the transplanted animals a new perichondral meniscus developed. After three months the transplants resembled normal menisci in size and thickness, while in the control animals only small rims of spontaneously grown tissue were seen. Microscopically, the perichondral menisci showed a normal orientation of collagen fibres and normal cellular characteristics, but in the central region, areas of calcification disturbed the regular tissue differentiation. Healing tissue in control animals lacked the normal fibre orientation and cellularity. SEM of perichondral menisci showed surface characteristics similar to those of normal sheep menisci without fissures and lacerations; the control specimens had these defects. The femoral and tibial cartilage in contact with the new menisci had normal surface characteristics apart from one animal with slight surface irregularities. Control animals showed superficial lesions after three months which increased at six to 12 months postoperatively. Microangiography of the newly grown tissue demonstrated a less intense vascularisation after three months when compared with normal menisci.

The failure stress and tensile modulus of perichondral menisci were significantly lower than those of normal contralateral menisci, and spontaneously regenerated tissue in meniscectomised animals had even lower values. There were no significant differences in values between newly grown perichondral menisci and spontaneously grown tissue.

In 20 skeletally mature female merino sheep, divided into four groups, we performed total medial meniscectomy, removal of the middle third of the patellar tendon, and tenotomy of the calcaneal tendon of the right hind leg. Group I (control) had no additional procedures. In the other three groups the medial meniscus was replaced by the middle third of the patellar tendon from the ipsilateral knee. The animals were killed at three (group II), six (group III), or 12 months (group IV) and the tendon-meniscus examined macroscopically, by light and scanning electron microscopy, and biomechanically. Remodelling of the tissue had taken place by 12 months but the failure stress and tensile modulus for the tendon-meniscus were lower than for the normal meniscus. Our evidence suggests that, in sheep, replacement of a meniscus by a tendon autograft may decrease the severity of the degenerative changes that occur after meniscectomy.