This experimental study aimed to determine the pattern of load transmission to the cement mantle and to the outer surface of six composite femurs implanted with three different designs of polished, collarless, tapered stems (2 specimens for each type), before and after the removal and reinsertion of the same stem. Strain distribution was measured with uni-axial and tri-axial strain-gauges before, after implantation and after reinsertion of the stems. Additionally, axial and rotational stability of the stem relative to the cement mantle and to the composite were determined by means of one extensometer and two linear variable displacement transducers (LVDT’s). All specimens were loaded simulating single leg stance of 3,25 body weight for a 708N subject. Static load were applied to the intact femurs, after implantation, after cyclic loads of 1Hz up to 3x10 5 cycles followed by 7Hz up to 1million cycles, and after reinsertion. Variation of strain and micro-motion during static loading following reinsertion were determined by the average of 10 cycles, with each cycle being represented by the difference between maximum and minimum values.

Linear regression analysis of the strain values obtained by the sensors in the cement mantle after reinsertion plotted with regard to the strains obtained initially by the same sensors before and after cyclic loading showed coefficients R2= 0.95; 0.91,with slopes of 1.12 and 1.03 respectively. The values of static strain of all sensors plotted with regard to values of initial static strain showed a very strong positive correlation (R2 = 0.98; slope = 0.96).

These findings support the concept that reinsertion of same design and size polished, collarless tapered stems may not alter the pattern of load transmission and stability at the interfaces between stem/cement, and cement/bone to the outer surface of bone. The mechanical conditions at the interfaces are restored with no need for additional cement during reinsertion if the cement envelope is preserved. The same mechanical principle that maintains the stability of the stem during subsidence for ”force-closed” fixation, may keep the stability of the interface following reinsertion. For these reasons this procedure may not be applicable to designs with texturing or pre-coating, and cylindrical-collared designs because in such conditions (“shaped-closed “fixation) the mechanics of stem/cement interface may not be restored.

Introduction: Treatment of hip joint disease with Total Hip Arthroplasty (THA) is in continuous evolution with new approaches, new size of components and type of bearing surfaces. Meanwhile, the analyses of proven implants continues to yeld results with a high survivor-ship and almost absent osteolysis, even with the greater wear found in the poliethylene (PE) insert of 10 to 15 year old implants. The results of these THA and the need to revise some implants not loosened but with wear of the PE, is the object of this study.

Methods: 330 non-cemented THA with the CLS expansion cup, performed by four surgeons over 15 years, are retrospectively analyzed. General anthropometric data of the studied population was obtained from medical records. Clinical elements are evaluated, among others the diagnosis, size and orientation of the components, time of partial/total weight bearing, period of crutch assisted walking, subjective patient satisfaction.

X-rays were measured for numerical assessment of the orientation of the cup, migration, radiolucent lines, osteolysis and indirect evaluation of the wear of the PE insert. Obtained data recorded over the life of the implant was computed for correlations. Other studied elements include survival rate, complication, occurrence and type of revisions.

Results: The encountered populations age was 62.7 +/− 10.9 years (Min 20, Max 86), the sex 43.1% male 56.9% female, the affected side 53.5% right 46.5% left. Charts of clinical elements, radiological measurements and correlation studies are presented. The wear of the PE insert correlates with the age of the implant and the inclination of the cup but not with radiolucent lines or osteolysis, which have a very low incidence. The complications and the survivorship analysis of the implants are presented and discussed. The encountered revision rate was 2.3% in the studied population. Without loosening, most revisions consisted on simple PE and head exchange.

Conclusions: In the authors’ opinion, the very high rate of long-term survivorship with the CLS expansion cup makes this an excellent implant for THA. Even in the 10 to 15 year old group of implants presenting greater wear of PE, there is very low presence of osteolysis and low rate of loosening, permitting to perform simple exchange of the insert and head, a simple revision procedure that will allow quick functional recovery, high patient satisfaction and a good cost/efficiency relationship.