Focal knee resurfacing : a translational study in sheep on the treatment of focal condylar cartilage lesions with metal implants

Background

The translational process of introducing a new treatment concept in medicine from innovation to clinical application is a challenging endeavour that often involves animal experiments. Focal cartilage lesion remains a clinical challenge and is considered to portend osteoarthritis (OA). For the last 30 years biological treatment strategies have aimed to regenerate a durable cartilage repair tissue. In the last decade a novel treatment strategy has been proposed by surgically replacing the void of the cartilage defect with a metal implant, focal knee resurfacing with metal (FKRM) implants.



Methods

We developed a double-radii, double-coated implant with an individualised instrumentation device for optimal positioning. This concept was tested in a larger animal model (sheep) evaluating the biological safety and efficacy of the implant. A total of 37 ewes were used in the experiments of this thesis. Implant position was measured using a developed laser-scanning protocol. General animal health, general cartilage health and macroscopic as well as microscopic cartilage evaluation were assessed according to a modified Mankin score as recommended by Osteoarthritis Research Society International (OARSI) for histological assessment of osteoarthritis in sheep. Osseointegration was evaluated histomorphometrically. A proposed classification of cartilage health adjacent to an implant was presented and implemented.



Results 

Osseointegration was excellent, bone-to-implant contact was measured with a mean (95% confidence interval – CI) of 90.6% (79-102) at six months and 92.3% (89-95) at twelve months, respectively. Implant position correlated to opposing tibial cartilage damage, an implant should not protrude. Microscopic score as a function of implant position showed a linear relationship (P = 0.008) such that Mankin score increased by 4.3 units (95% CI: 1.5, 7.0) per each mm elevation in implant height. Using a dedicated guide the general implant position was consistent and adequate, and was on average 0.54 mm recessed (95% CI: 0.41, 0.67). Cartilage damage of the medial tibial plateau opposing the implant was increased compared to the non-operated knee by 1.77 units (P = 0.041; 95% CI: 0.08, 3.45) on a 0-27 unit scale. Remaining joint compartments were unaffected. Cartilage health adjacent to the implant was satisfactory and Hydroxiapatite (HA) improves hondrointegration. Untreated critical size cartilage defects did not heal at six-month follow-up.



Conclusions

Focal knee resurfacing with metal implants is a safe and viable treatment option for full depth focal condylar cartilage lesions. Cartilage damage evaluated microscopically was acceptable given implant position related to surrounding cartilage was optimal. Implant position is of utmost importance and individualised implants and instrumentations devices are recommended. An implant should never sit proud. Clinical studies of symptomatic focal cartilage lesions are recommended but prior to prophylactic treatment of asymptomatic patients more research is needed