Abstract
A good primary mechanical stability is a prerequisite for secondary cementless stem fixation by osseointegration. The construction, basic shapes and their development, as well as the nature of the surface in the CLS system are thus directed towards this goal.
The biomechanical concept of the CLS stem is characterised by the three-dimensional V-form, the strongly accentuated ribs in the proximal region and the all-round conical form of the distal part of the stem. The construction uses the adaptable trabeculum for the primary fixation. The features that characterise the CLS stem are found both in the classical version and the varus version.
The fixation of the stem in the proximal area should guarantee that transmission of force is retained within this area. The interposition of the cancellous structures reduces the danger of the bond between the bone and prosthesis becoming stiff. On introduction, the axial ribs exercise a cutting action and promote stability. In order to avoid tension peaks, the surfaces of the distal part of the stem are well rounded. The shape of the distal part of the stem is kept relatively slender, thus direct contact with the cortex is avoided. This means reduced incidence of pain for the patient, plus the new trabecular structures can form between the cortex and the implant, which eventually result in osseointegration.
To what extent and how quickly an implant becomes integrated is not just a question of primary stability. Important factors that have an influence also include the material and the surface structures of the prosthesis. It is generally the case that new bone grows preferentially on protruding parts such as prosthesis ribs or edges, and also on existing trabeculae, such as those altered by the surgery. The formation of new bone tissue in association with the CLS prosthesis was thoroughly investigated in many histological studies. The coarse-grained titanium alloy is, to a great extent, osteophilic. This is demonstrated by the fact that bone marrow or new bone can form directly on the surface of the CLS prosthesis.
The indications can be calculated on the basis of four key parameters: age, sex, the degree of osteoporosis, and the morphology of the femur. The morphological-cortical index (MCI) is used as an instrument to determine the shape of the femur. This provides a reliable method of classification of one of three basic morphological types: trumpet-shaped, cylindrical, and dysplastic.
The first 300 consecutive primary total hip arthroplasty (THA) procedures (299 patients), from Dec. 1983 to Apr. 1985, using a collarless, three-dimensional tapered, straight, titanium alloy stem with a grit-blasted surface, were evaluated clinically and radiographically in up to 16 years of follow-up. Radiographic evaluation assessed Engh’s implant-bone femoral fixation score, implant-bone demarcation, and periprosthesis osteolysis. The average duration of long-term radiographic follow- up was 12.6 years (10–16 years).
Patients were evaluated clinically by the HHS system. At last examination, five hips were lost to follow-up and 69 patients had died. The femoral revision rate was 7% (two aseptic loosening, five septic, 12 osteolysis). Femoral component survivorship was 95% at 10 years and 90% at 14 years. Femoral implant-bone fixation was stable, bone ingrowth in 97% stable, fibrous fixation in 1% and unstable, fibrous-fixation in 2% of the cases revised for acetabular mobilisation.
The grit-blasted, press-fit, collarless, tapered femoral component continued to perform well. While considering a “first-generation” cementless stem, CLS is still in use without virtually any design modifications.
The abstracts were prepared by Mrs Dorothy L. Granchi, Course Coordinator. Correspondence should be addressed to her at PMB 295, 8000 Plaza Boulevard, Mentor, Ohio 44060, USA.
