We developed an in vivo model of the attachment of a patellar tendon to a metal implant to simulate the reconstruction of an extensor mechanism after replacement of the proximal tibia. In 24 ewes, the patellar tendon was attached to a hydroxyapatite (HA)-coated titanium prosthesis. In 12, the interface was augmented with autograft containing cancellous bone and marrow. In the remaining ewes, the interface was not grafted.

Kinematic gait analysis showed nearly normal function of the joint by 12 weeks. Force-plate assessment showed a significant increase in functional weight-bearing in the grafted animals (p = 0.043). The tendon-implant interface showed that without graft, encapsulation of fibrous tissue occurred. With autograft, a developing tendon-bone-HA-implant interface was observed at six weeks and by 12 weeks a layered tendon-fibrocartilage-bone interface was seen which was similar to a direct-type enthesis.

With stable mechanical fixation, an appropriate bioactive surface and biological augmentation the development of a functional tendon-implant interface can be achieved.

We investigated the implant-bone interface around one design of femoral stem, proximally coated with either a plasma-sprayed porous coating (plain porous) or a hydroxyapatite porous coating (porous HA), or which had been grit-blasted (Interlok). Of 165 patients implanted with a Bimetric hip hemiarthroplasty (Biomet, Bridgend, UK) specimens were retrieved from 58 at post-mortem.

We estimated ingrowth and attachment of bone to the surface of the implant in 21 of these, eight plain porous, seven porous HA and six Interlok, using image analysis and light morphometric techniques. The amount of HA coating was also quantified.

There was significantly more ingrowth (p = 0.012) and attachment of bone (p > 0.05) to the porous HA surface (mean bone ingrowth 29.093 ± 2.019%; mean bone attachment 37.287 ± 2.489%) than to the plain porous surface (mean bone ingrowth 21.762 ± 2.068%; mean bone attachment 18.9411 ± 1.971%). There was no significant difference in attachment between the plain porous and Interlok surfaces. Bone grew more evenly over the surface of the HA coating whereas on the porous surface, bone ingrowth and attachment occurred more on the distal and medial parts of the coated surface. No significant differences in the volume of HA were found with the passage of time.

This study shows that HA coating increases the amount of ingrowth and attachment of bone and leads to a more even distribution of bone over the surface of the implant. This may have implications in reducing stress shielding and limiting osteolysis induced by wear particles.

The use of extendible distal femoral replacements is a relatively new treatment alternative for malignant bone tumours in growing individuals. Although their appearance was widely appreciated, questions about functional practicality and longevity remain unclear. With longer follow-up, advantages of immediate functional restoration and beneficial psychological aspects seem to be overshadowed by an increase in complications such as aseptic loosening, infection or prosthetic failure.

We have reviewed 18 children with such tumours who were treated between 1983 and 1990 by custom-made Stanmore extendible distal femoral replacements. Four died from metastatic disease within 2.5 years of operation and two required amputation for local recurrence or chronic infection.

The remaining 12 patients were followed for a mean of 8.7 years (6 to 13.2). A mean total lengthening of 5.2 cm was achieved, requiring, on average, 4.3 operations. Using the Musculoskeletal Tumor Society rating score the functional result at review was, on average, 77% of the expected normal function, with seven patients achieving ≥ 80%. Revision of the prosthesis was required in ten patients, in six for aseptic loosening, at a mean of 6.2 years after the initial procedure.