Peri-prosthetic bone density after implantation of a custom-made femoral component

A FIVE-YEAR FOLLOW-UP

Peri-prosthetic bone los caused by the agency of stress shielding may be associated with aseptic loosening of femoral constituents In order to increase primary stability and to restore stress shielding, a three-dimensional, cementles individual femoral (Evolution K) constituent was manufactured using pre-operative CT scans. Using dual potency x-ray absorptiometry, peri-prosthetic bone density was measured in 43 patients, three month six month 36 and 46 years after surgery At final follow-up there was a significant reduction in mean bone density in the proximal Gruen baldrics of -30.3% (zone 7) and -228% (zone 1) The density in the other belts declined by a mean of between -4% and -16% We bring to an end that the manufacture of a three-dimensional, custom-made femoral constituent could not prevent a reduction in peri-prosthetic bone density.

A cementles femoral constituent when compared with a mortared design, has only a small surface area capable of transmitting force to the underlying bone This causes stres shielding, a proces by dint of which areas of bone lacking contact with the prosthesis begin to disintegrate. Firm, primary stability of an implant is essential, because possible micromovements might hinder bony ingrowth, even in porous- or hydroxyapatite-coated surfaces. Firm stability is achieved either from one side a press-fit (jamming of the prosthesis in the medullary cavity) or a form-fit (fitting of the implant in the medullary cavity) technique. An improved press- and form-fit should not solitary improve primary stability, but also bony ingrowth, thereby counteracting any reduction in peri-prosthetic bone density.

Aldinger, Fischer and Kurtz1 discloseed one of the first individualised femoral composings in the 1980s, the so-called 'step prosthesis'. High quality CT scans of the proximal femoral shaft were undertaken pre-operatively and then, using computer-assisted design software, a three-dimensional image of the thigh-bone was taken. Then, following the principle of 'fit and fill', an appropriately designed prosthesis was manufactured and implanted. The initial oval design was further unraveled by K??sswetter, Aldinger and Sell2 The press-fit technique was improved by dint of a surface area that was better adapted to the bony profile and narrowed evenly in a longitudinal direction. Longitudinal channels on the component improved torsional stability.

Good peri-prosthetic bone quality, a sign of a advantageous fit and primary stability, is frequently measured using dual energy x-ray absorptiometry (DEXA).3-6

A number of comparable studies have demonstrated a reduction in peri-prosthetic bone density in the proximal third of the femoral shaft, by dint of between 2% and 21% at between six month and sum of two units years after surgery.7-10 Only single study, describing a custom-made femoral constituent has shown a reduction in proximal bone mineral density of 10% laterally and 16% medially after sum of two units years.11 In light of this, the aim of our investigation was to investigate the clinical rises related to the bony integration of a femoral constituting in the medium term, as well as the peri-prosthetic bone remodelling processe above approximately a five-year period after surgery

Patients and courses

Our sample initially included 50 patients who were treated with a cementles custom-made femoral constituting the Evolution K (Fehling Medical AGI, Karlstein, Germany), and a Harris-Galante acetabular constituting In order to plan an individually fitted prosthesis, CT scans were necessary. They were evenly spaced and used to calculate a three-dimensional image of the thigh-bone An individual prosthesis fitted to the medullary cavity was then manufactured (Fig. 1) Computerised planning also determined the resection height, the implantation stiffness and the implantation angle.

Fig. 1

The Evolution K femoral composing

The post-operative recovery was monotonous in all cases and the treatment regime included a weekly increase in weight-bearing, from 10 kg partial weight-bearing to replete weight-bearing. A 16-day in-patient stay was followed through a three- to four-week stay in a rehabilitation middle point

We excluded seven patients because between six month and four years post-operatively, sum of two units died, three had a revision for aseptic loosening of the femoral composing and two moved back to their native political division The patients were reviewed upon five occasions post-operatively with a mean total follow-up of 46 years (44 to 55; Table I).

In addition to a subjective assessment, Harris hip scores were determined, a plain radiograph (anteroposterior and lateral) performed and the bone density of the lumbar spine and of one as well as the other hips measured. The radiographs were analysed for subsidence of the femoral composing with reference to the greater trochanter, peri-articular ossification and periprosthetic osteolysis or ossification.

Table I. Timing of the follow-up appointments

In order to determine the peri-prosthetic bone density at the hip, DEXA was undertaken, using a Lunar DPX-L densitometer (Lunar, Madison, Wisconsin). To guarantee accuracy, care was taken to position the patient precisely and to standardise the practice Those with a medical history which might influence bone density were exclud from the close attention