Experimental Coxofemoral Replacement Hemiarthroplasty in the Pony

For the in vivo evaluation of hip joint prostheses that are developed for man, the suitability of the pony as an animal model has been studied. Human and custom made prostheses were implanted into 1 femur in each of 7 ponies to replace the femoral head. The surgical approach was through a caudolateral incision. The prosthesis was fixed in the femur with bone cement. Because of errors in the implantation technique and resulting lack of sufficient fixation 2 ponies as well as 1 control animal with an excision arthroplasty were unwilling or unable to stand. Rigid fixation of the implant was achieved in 3 ponies. They were returned to pasture after 8 days. A full return to a normal gait did not occur within an observation period of up to 3 months. All 3 ponies developed a dorsolateral coxofemoral luxation and pseudarthrosis within the first 10 days. A mismatch between the equine anatomy and human prosthesis geometry was thought to be the cause. We, therefore, believe that the pony does not provide a suitable model for the testing of human femoral joint implants. The results, however, indicate that coxofemoral surgery can be performed in the adult pony. For clinical reconstruction of joint function, a custom made total artificial hip joint might provide better results.