Design of a prosthetic anterior cruciate ligament

Injury to the knee can have a dramatic effect on a person's lifestyle. Participation in sports and exercise, general mobility, and even commonplace activities like walking up stairs can all be restricted or prevented completely if knee joint function is impaired. A common form of knee injury, estimated to affect roughly one in three thousand people per year, involves damage to the anterior cruciate ligament (ACL). The ACL passes through the centre of the knee, connecting the femur ('thigh bone') and tibia ('shin bone'). Rupture of the ACL can lead to instability of the knee, potentially resulting in knee pain, limitation of mobility and long term joint damage.This thesis describes the design and preliminary testing of a prototype prosthetic ACL intended to restore natural knee motion after ligament injury. The functional characteristics of the human ACL are established, and the major failure modes of previous prosthetic ligaments are identified as fatigue, abrasion, creep and joint inflammation due to particulate wear debris.The design of a ligament prosthesis with multiple functional components, the aACL (artificial ACL), is outlined. The major functions of the prosthesis (supporting the required loads, possessing the correct elasticity, preventing abrasion and achieving sound bone anchorage) are separated and provided by distinct structures. Alternative design options for the different components are presented and suitable materials determined. The high strength polymer materials Spectra, Kevlar and Vectran are selected for investigation for use as the load bearing component of the prosthesis. A braided polypropylene sleeve is used to protect this component from abrasion. An elastic system based on either disc springs or an elastomer block loaded in compression is specified to replicate the stiffness of the natural ACL. Techniques available for anchoring solid prosthetic materials to bone are reviewed. Titanium is chosen for the anchor material as it has been shown to promote strong fixation through bone ingrowth. A threaded anchor profile designed to promote bone densification and achieve high fixation strength without cement is specified.The second part of this thesis describes the experimental evaluation of the design proposals. The various methods of laboratory testing which have been applied to previous ligament prostheses are examined, and their strengths and weaknesses established. A test plan is then proposed for the aACL in order to allow evaluation of its suitability for surgical use. Design options for a test facility to determine the resistance of the aACL to repeated knee flexion and extension are reviewed. The design procedure and methodology used to develop a test facility are discussed, and the final design presented. The construction and initial trials of the equipment are outlined. The endurance, creep and tensile tests which were performed on the Spectra, Kevlar and Vectran ligament materials are reported.The test conditions are described and qualitative observations and quantitative results of the effects of cyclic testing on the samples are presented. It is concluded that of the three materials tested, Spectra is the most suitable for use in a prosthetic ACL as it possesses the greatest resistance to fatigue and abrasion damage and the lowest rate of fibre shedding. The titanium anchor design was evaluated experimentally using pull-out tests of prototype anchors in pig bone to obtain estimates of the in-use anchorage strength. The anchorage strength achieved at the time of implantation is high enough to permit immediate patient mobility after surgery. Long term anchor strength will need to be assessed using in vivo trials. The thesis concludes by recommending that further development and testing be conducted to fully demonstrate the suitability of the aACL. A programme of further laboratory trials to continue the development of the prosthesis towards eventual surgical use is proposed