[A ceramic total hip endoprosthesis for implantation without bone cement. Preliminary report (author's transl)]

Strain adaptive bone remodelling in total joint replacement

Histomorphologic analyses of artificial joint components implanted into bone need special technology for processing and for documentation; published histological work systematically done therefore is rare. The histopathology, three-dimensionally analyzed in a complete sequence of sections is, however, the only precise answer in terms of biocompatibility and bone response. A complete analysis allows a type-related predictable prognosis of an implantation that is at least comparable to a finite element analysis with respect to load transfer to host bone. The histopathologic collection of the ZOW Munich is comprised of more than 5000 nondemineralized bone and joint specimens and more than 500 artificial joint components implanted in the human skeleton for up to 25 years. Fifty-nine implant-bone specimens without signs of loosening already have been processed and analyzed systematically. According to the strain-adapted bone remodelling, different types of anchorage clearly were differentiated and their morphologic substrate could be worked out. Based on that, the cemented standard anchorage could be distinguished histologically from the cemented press-fit procedure, and the noncemented press-fit from the porous ingrowths pattern. In terms of the topography of the bony integration, the proximal and distal press-fit and ingrowth pattern were analyzed; beside that, the cemented and noncemented epiphyseal resurfacings could be defined histologically. In all histologic specimens the remodelling appeared as a result of stress-related strain, reflecting stiffness of the implant and the resistance of bone to deformation. It clearly was worked out that all success of cemented components is based on preserved cancellous bone honeycombs stiffened by bone cement, representing an adaptation of bone in terms of stiffness to the stiff implants.

Morphometric examination of straight, tapered titanium stems: a retrieval study

Cementless straight, tapered rectangular cross section titanium stems have been used by the senior author since 1979. Thirty-four stems retrieved postmortem, after between 10 days and 15.2 years in situ, were studied morphometrically and histologically. Nineteen stems were first generation (1979-1986), with an incomplete sagittal taper and a mean surface roughness of 1 microm (Ra 1.23 +/- 0.3 microm); 15 stems were second generation after 1986, with a full biplane taper and a mean surface roughness of 4 microm (Ra 4.14 +/- 0.36 microm). Implant surface bony coverage was determined morphometrically in 10 segments of the stems, and expressed as a bone implant contact index. Histologically, there were no differences between implants with different levels of roughness. Morphometrically, the first-generation stems showed significant differences in coverage (distal > proximal); second-generation stems had a more uniform pattern. Stems retrieved early after arthroplasty had a mean bone implant contact index of 10%. The mean bone implant contact index showed attainment of maximum coverage by 5 years after arthroplasty, without additional apposition or loss thereafter. Patients younger than 65 years at arthroplasty had similar bone implant contact indices to patients 65 years or older; coverage in the six patients 80 years of age at retrieval did not differ from the rates in the other patients. Morphometry was able to provide objective evidence of design change effects. No differences in coverage were found in terms of times in situ, patient age at arthroplasty and at retrieval, and degree of stem surface roughness.

Alumina ceramic bearings for hip endoprostheses: the Austrian experiences

The current authors review clinical and retrieval experiences with hemispheric monolithic alumina ceramic sockets (Group 1), implanted between 1976 and 1979, and similar modular titanium sockets with alumina ceramic inlays (Group 2), implanted between 1990 and 1995. Both cementless sockets articulated with alumina ceramic femoral ball heads for total hip joint replacements. Clinical followup of patients with hemispheric monolithic alumina ceramic sockets (Group 1, 138 sockets) resulted in a total failure rate of 19.6% after 5 to 20 years. Radiologic analysis of eight stable sockets showed migration of 0.2 mm to 2.89 mm, but in four sockets at risk for late aseptic failure after an average followup of 12.5 years as much as 13.4 mm of migration was seen. Histologic evaluation revealed pseudosynovial membranes as thick as 1 mm with fine birefringent wear particles within mononuclear macrophages around two stable retrieved sockets. The membranes around four loose sockets were 6 to 10 mm thick and also heavily loaded with larger alumina wear particles. After 7 years followup clinical analysis of patients with modular titanium sockets with alumina ceramic inlays (Group 2, 30 sockets) resulted in four revisions, compared with one revision of 50 identical sockets (control group) with polyethylene instead of alumina ceramic inlays. Wear particle analyses in scanning electron microscopy showed significantly more particles (x 10(9) +/- standard deviation/g dry tissue) from the control group (4.26+/-6.38), compared with alumina ceramic bearings of Group 1 (0.70+/-0.79), and of Group 2 (1.62+/-2.13). The alumina particle sizes ranged between 0.13 and 78.38 microm. The mean annual linear wear of 38.8 microm was calculated for the bearings in Group 1, and of 26.94 microm for bearings in Group 2. These results support the good tribologic and biologic performance of alumina ceramic bearings for total hip arthroplasty.

Concept and material properties of a cementless hip prosthesis system with Al2O3 ceramic ball heads and wrought Ti-6Al-4V stems

A cementless total hip system of modular design is presented. The design of the stem provides for a firm seating on the cortical bone over a large area. Therefore primarily stable fixation is accomplished. The stem in seven different sizes is made of hot forged Ti-6Al-4V Protasul-64 WF alloy of high corrosion fatigue strength. The Biolox ceramic ball with three different neck lengths is connected to the high strength stem by means of a special selflocking conical spigot with a structured surface. The ceramic ball is combined with a polyethylene screw socket in four different sizes, designed by Endler and anchored in the acetabulum without acrylic cement. This hip prosthesis system has proved itself in clinical application since 1979.

[Comparative experimental investigations with bioglass (L. L. Hench) and Al2O3-ceramic coated with mod. bioglass. II. Results of experiments with loaded implants (author's transl)]

This report is based on 4 comparative experimental series is sheep using a cementfree total hip replacement model. Basically a smooth stem design was compared to a step stem design for biomechanical interlocking anchorage. Both designs were implanted as pure high density Al2O3-ceramic components and as Al2O3-ceramic parts coated with Bioglass or Bioglass-ceramic (post op. follow up 13 months, 17 animals). None of the 4 stem modifications showed a tendency for permanent stabilisation or bonding in bone. Besides insufficient bonding of the glass coatings to the substrate and apparent biodegradability of the bioglass coatings in the body, insufficient biomechanical knowledge of endosteal direct anchorage of prosthetic devices is the main reason for failure in these experiments. An exclusive biochemical way of anchoring prosthetic parts in the marrow cavity is rejected. The results presented are discussed in the light of the recent literature.