Introduction

Cementless fixation in unicompartmental knee replacement (UKR) is a developing field with advances in implant design and fixation surfaces. Although cementless options in unicompartmental replacement have been available for nearly 20 years, the operation has not become widespread. Cemented fixation is used in the majority of UKR procedures, with good clinical results. However, there is a concern over the suitability of cemented fixation in the younger, high-demand patient. Cementing in UKR is difficult and cementing errors, such as cement loose bodies, are common and may cause failure. In addition, the slow development and general acceptance of cementless UKR may be in part due to poor results from early designs. The use of cementless fixation in UKR is likely to increase with the aim for long-term biologic fixation being a realistic goal.

Rationale

The development of cementless UKR has mirrored that of total knee replacement (TKR), with the use of porous coatings, with and without hydroxyapatite, and, in some designs, the addition of cancellous screw augmentation. All designs have utilized a keel on the tibial component, although the shape, size, and direction have shown considerable variation (Figs. 12–1 and 12–2). The femoral component has usually mirrored the cemented equivalent for each design, although those femoral designs with a single peg had a second peg added for rotational stability.

Figure 12–1 A cementless Oxford tibial component demonstrating a porous titanium layer with a hydroxyapatite coating.

Figure 12–2 A cementless, hydroxyapatite-coated Unix tibial component with a horizontal keel and screw holes for additional fixation.

(Photograph courtesy of Jean-Alain Epinette.)

One of the first advocates of cementless UKR was Jean-Alain Epinette,¹ who designed and developed a hydroxyapatite-coated tibial component. This component differed from most designs by employing a horizontal keel that slotted beneath the tibial spines, thus maintaining bone stock beneath the tibial tray. In addition, there were cancellous screws to augment the initial fixation. Kaiser and Whiteside further highlighted the importance of initial stability in 1990, with a cadaveric in vitro study demonstrating the additional stability provided by cancellous screws in a unicompartmental tibial component, compared with a posteriorly angled peg.²

Cementless fixation has many potential advantages over the more commonly used cemented fixation. In hip arthroplasty, cementless fixation has become ever more popular, whereas cementless total knee arthroplasty has remained less popular, particularly with regard to the tibial component. This is probably due to the increased difficulty in obtaining good initial fixation with a mostly flat tibial component resting on a flat tibial plateau. Although cementation provides an adequate fixation method in most cases, the use of cement results in several consequences that encourage perseverance with the development of cementless fixation. The disadvantages of cement use include possible thermal damage to the bony surfaces, an increased operative time, and the risk of cementing errors such as cement loose bodies or incorrect cement loose bodies, incorrect seating or impingement on cement. Cementless fixation avoids these problems and has the advantage of preserving tibial bone, as well as the theoretical potential of very long-term biologic fixation. However, cementless fixation in total knee arthroplasty has struggled to gain widespread acceptance due to some poor results, with a particular risk of failure of ingrowth.

Cementless fixation in UKR has mainly been used in the few centers that are strong advocates of unicompartmental surgery. The latest reports from the Australian and New Zealand joint registries show that cementless UKR accounts for 11% and 6% of all UKR procedures, respectively.^3,4 The Swedish register reported that all the UKRs performed in 2009 had both components cemented.⁵ Neither the National Joint Registry of England and Wales nor the Canadian registry gives information regarding fixation method for UKR.^6,7

Although the presence of thin, nonprogressive, radiolucent lines (Fig. 12–3) has long been recognized in knee arthroplasty, with no associated increase in failure rate, the exact cause and effect of these lines is still unknown. There is, however, a perception that a radiolucent line is indicative of suboptimal fixation. The incidence of radiolucency differs beneath different implants, suggesting that the mechanical environment is important in their development. Forsythe et al.⁸ reported an incidence of over 50% with the cementless Whiteside Ortholoc II UKR, whereas Pandit et al.⁹ reported just 7% of cementless Oxford tibial components had a partial radiolucency, with none with a full radiolucency. There is also a reported increase in incidence of radiolucent lines with cemented Oxford components compared to the cementless version.⁹ There is no clear consensus on the role or importance of thin (≤1 mm), stable, so-called physiologic radiolucent lines, although there is evidence that they are not associated with either loosening or a decrease in clinical outcome scores. However, the greatly reduced incidence in cementless UKR is encouraging and suggests good bony fixation (Fig. 12–4). The Oxford UKR is particularly suited to cementless fixation because of the fully mobile bearing. The bearing movement causes a marked reduction in shear forces being transmitted through the tibial tray, and therefore the tray is mostly subjected to compression. This is an almost ideal mechanical environment for cementless fixation. Radiostereometric analysis studies of cementless fixation in TKR have shown a consistent pattern of migration, which differs from cemented fixation. Cemented components usually demonstrate continuous early migration, but the magnitude is small and stability is achieved by 2 years. In contrast, cementless components usually migrate a larger amount in the first few months before stabilizing. Onsten and Carlsson have demonstrated a reduction in the movement between 1 and 2 years postoperatively with the addition of a layer of hydroxyapatite to a porous-coated PFC tibial component.^10,11 Likewise, Regner et al. demonstrated the reduction in maximum total point movement at 5 years in the Freeman-Samuelson TKR tibial component with the addition of hydroxyapatite to the undersurface.¹²

Figure 12–3 A physiologic radiolucency beneath a cemented Oxford tibial component.

Figure 12–4 A well-seated cementless Oxford tibial component demonstrating no radiolucency at 5 years.

Indications

The indications for UKR are controversial and vary according to the surgeon’s philosophy. At Oxford, our view is that there should be full-thickness cartilage loss on both the medial tibial plateau and femoral condyle (bone-on-bone osteoarthritis), any intra-articular varus deformity should be correctable, and the anterior cruciate ligament (ACL) should be intact. Patello-femoral joint damage is ignored unless there is severe damage laterally. Cartilage damage on the lateral femoral condyle is permissible provided there is full-thickness cartilage present on a valgus stress radiograph and there is no full-thickness loss centrally. Full-thickness damage on the medial side of the lateral femoral condyle is not a contraindication.¹³