Introduction

Hemiarthroplasty is an option for the treatment of unicompartmental osteoarthritis. The implants designed by McKeever and MacIntosh more than 50 years ago provided the intellectual and clinical basis for interpositional hemiarthroplasty. Although these approaches had generally good clinical outcomes, the technical difficulty associated with their use and the advent of very successful total knee arthroplasty led to their general abandonment among most orthopaedic surgeons. However, the challenges of treating younger, more active patients and the desire to adopt minimally invasive and bone-preserving techniques have given rise to the development of interpositional devices (iPDs) that represent an evolutionary change from the early versions of such devices. This chapter gives an overview of spacer devices designed for the operative treatment of unicompartmental osteoarthritis. The design and conceptual approach of each of the current options are reviewed.

Early Development of Hemiarthroplasty

For patients with osteoarthritis limited to a single femoral-tibial compartment, the concept of a metallic hemiarthroplasty has a long history. The genesis of the concept of using an iPD, in which a material is placed between the condyles within the joint to reduce wear or prevent adhesion, goes back almost 150 years, when it was first suggested by Verneuil.¹ Over the ensuing years, a wide variety of materials were tried, ranging from chromicized pig bladder in 1918 to vitallium in 1940. In 1960, McKeever² described a metallic prosthesis that was designed to be placed in the femorotibial compartment and fixed to the tibial condyle (Howmedica, Rutherford, NJ) (Fig. 4–1). This design was subsequently modified by MacIntosh. Metallic hemiarthroplasty was introduced into orthopaedic practice in the 1950s and 1960s by McKeever² and MacIntosh.³ MacIntosh and Hunter described the hemiarthroplasty approach as follows: “The aims of hemiarthroplasty are to correct the varus or valgus deformity by inserting a tibial plateau prosthesis of appropriate diameter and thickness to build up the worn side of the joint and thus to restore normal stability of the knee, to relieve pain and to improve function and gait. The collateral ligaments usually maintain their own length in spite of long-standing varus and valgus deformity, and stability is maintained by a prosthesis that is thick enough to correct the deformity and to take up the slack of the collateral ligaments.”⁴

Figure 4–1 McKeever hemiarthroplasty implant.

(Courtesy of Howmedica, Rutherford, NJ.)

Reports of early experience with both of these devices were generally encouraging. The two approaches differed primarily in the method of fixation. The McKeever implant had a keel that was inserted into the tibial condyle to provide mechanical fixation. In contrast, the MacIntosh implant was “held in position by the anatomy of the knee joint, and stability depends upon the taut collateral ligaments. No additional fixation is necessary. The top of the prosthesis has a contoured surface with rounded edges to provide the condyle with a permanent low-friction area. The undersurface is flat with multiple serrations to ensure a snug fit and stability.”⁴ In a series reporting 10-year follow-up for 75 MacIntosh implants, Wordsworth et al. found that 11 (14.7%) had been revised to arthroplasty and concluded that, although “greater angular deformities pre-operatively reduced the chance of success in the medium term, late failure of the arthroplasty after five years was very rare.”⁵