‘GAP HEALING’ OBSERVED IN UNCEMENTED HYDROXYAPATITE COATED PRIMARY TOTAL KNEE ARTHROPLASTY

Abstract

The success of uncemented arthroplasty depends on the achievement and maintenance of implant stability. Despite the use of modern instrumentation to obtain an accurate implant fit during total knee replacement, small gaps often remain visible at the bone-prosthesis interface on high quality fluroscopically-assisted radiographs. Although the clinical significance of these gaps is unclear, their presence delays bony fixation of the implant.

In uncemented total hip arthroplasty, hydroxyapatite costing has been used to enhance early stability of the implant: bony apposition has been shown to occur rapidly even in the presence of a small gap between the implant and the bone. In addition, recent RSA (Radio-stereo-photogrammateric analysis) studies have shown reduced micromotion and enhanced implant stability with hydroxyapatite coating of both hip and knee prostheses.

The following study was designed to observe and investigate the phenomenon of ‘gap-healing’ around hydroxyapatite coated uncemented total knee prostheses.

Over a 15-month period a hydroxyapatite coated uncemented total knee prosthesis was implanted in 99 patients undergoing 108 primary knee arthroplasties. The patients were prospectively reviewed at regular intervals with an average follow up of 18 months and a minimum of 12 months. The implant-bone interface was evaluated by obtaining serial fluroscopically-assisted radiographs.

On the immediate postoperative radiographs, small gaps between the implant and bone were seen in most knee. These gaps were visible on average in 2.16 AKS (American Knee Society)zones per knee. Most of the gaps were seen in Femoral zones 2,3,5 and Tibial zones 1 & 4. The majority of the gaps were under 1mm depth. Gaps> 2mm were seen only in 6 patients. Healing of the gaps was first seen at 3 months postoperatively, the average number of zones involved per knee dropping to 1.54. There was good evidence of ‘gap healing’ occurring at all the bone-implant interface zones. At the end of the first postoperative year, only 0.8 zones per knee were involved.2mm gaps remained visible in 3 patients.

In animal experiments, hydroxyapatitie coated porous surfaces have shown an increased the rate of bone ingrowth for as many as 52 weeks after implantation. In our study, progressive bone ingrowth and gap-healing has been observed beyond this period, the average involved zones on 2 –year radiographs being 0.4 per knee.

During the study period, the American knee score improved from 39.52 preoperatively to 89.97 at 1 year postoperatively. No relation was found between the clinical scores and the presence or absence of gaps on follow-up radiographs.

This study demonstrates the phenomenon of ‘gap-healing’ following uncemented hydroxyapatitie coated primary total knee arthroplasty in an unselected group of patients. Gaps under 1 mm at the implant –bone interface heal readily. Healing of gaps> 2mm is less predictable.