Key points

Introduction

Unicondylar knee arthroplasty (UKA) is now an established treatment for end-stage osteoarthritis of the knee and accounts for around 10% of all primary knee replacements. Very high early failure rates, largely caused by wear of polyethylene components, have been addressed by the use of improved polyethylene and developments in design, particularly the introduction of mobile bearings. UKA has advantages over total knee arthroplasty (TKA), including faster recovery times, reduced perioperative morbidity and mortality, and improved returns to work and sport, and there is now a significant body of literature supporting the use of UKA. Excellent outcomes (in terms of both implant survival and functional outcome) have been reported up to 20 years after implantation.

However, the revision rate of UKA remains a concern. National Joint Registries (NJR) report around a threefold increase in crude cumulative revision rate at 8 to 10 years for UKA compared with TKA, and this is supported by other observational studies that demonstrate significantly poorer survivorship for UKA.

The difference in revision rate between UKA and TKA is likely to be multifactorial. The ease of revision of UKA lowers the threshold for revision when compared with TKA ; patients for UKA are likely to be younger with higher demands, and UKA seems to be more sensitive to surgical inexperience. Alongside a greater understanding of these factors, modifications to implant design, particularly in terms of the bone-implant interface, have the potential to improve the reproducibility of UKA and, in turn, to improve the survivorship.

Introduction

Unicondylar knee arthroplasty (UKA) is now an established treatment for end-stage osteoarthritis of the knee and accounts for around 10% of all primary knee replacements. Very high early failure rates, largely caused by wear of polyethylene components, have been addressed by the use of improved polyethylene and developments in design, particularly the introduction of mobile bearings. UKA has advantages over total knee arthroplasty (TKA), including faster recovery times, reduced perioperative morbidity and mortality, and improved returns to work and sport, and there is now a significant body of literature supporting the use of UKA. Excellent outcomes (in terms of both implant survival and functional outcome) have been reported up to 20 years after implantation.

However, the revision rate of UKA remains a concern. National Joint Registries (NJR) report around a threefold increase in crude cumulative revision rate at 8 to 10 years for UKA compared with TKA, and this is supported by other observational studies that demonstrate significantly poorer survivorship for UKA.

The difference in revision rate between UKA and TKA is likely to be multifactorial. The ease of revision of UKA lowers the threshold for revision when compared with TKA ; patients for UKA are likely to be younger with higher demands, and UKA seems to be more sensitive to surgical inexperience. Alongside a greater understanding of these factors, modifications to implant design, particularly in terms of the bone-implant interface, have the potential to improve the reproducibility of UKA and, in turn, to improve the survivorship.

Rationale for cementless UKA

The commonest reason given for revision of UKA in NJR is aseptic loosening, accounting for up to 48% of all UKA revisions. This mechanism is significantly rarer in published series, and it seems to account for a significant proportion of the difference in UKA survival between these series and NJR data.

The higher rate of aseptic loosening in NJRs is replicated in series from lower volume centers and those at the beginning of their learning curve, but not in high-volume centers with experienced surgeons. Two main theories for this discrepancy have been suggested: errors in cementation technique and misdiagnosis of loosening.

Cementation is technically demanding, particularly in conjunction with the minimally invasive surgical (MIS) technique, which is now used commonly for UKA. Analysis of failures from units new to MIS UKA demonstrates that a high proportion of failures are due to the inability to achieve an adequate cement mantle. Cadaveric studies have demonstrated very poor cement penetration in UKAs implanted with inadequately prepared surfaces. Removal of loose fragments of cement is particularly difficult with the MIS technique, and such fragments seem to cause intra-articular impingement, leading to excessive wear and subsequent implant loosening, which is apparent in specimens retrieved from revised UKAs.

Misinterpretation of postoperative radiographs is a common problem. Radiolucent lines are a common finding adjacent to cemented UKAs. These “physiologic” radiolucencies are narrow and nonprogressive and represent an incomplete layer of fibrocartilage at the cement-bone interface ( Fig. 1 ). Although they may indicate suboptimal fixation, they are not associated with symptoms, have a poor specificity and sensitivity for loosening, and do not affect survival. Anteromedial tibial pain is an occasional finding following UKA (probably as a result of increased proximal tibial stress) and usually resolves within the first year with conservative treatment. It has been hypothesized that the combination of radiolucent lines and anteromedial tibial pain may be misinterpreted as indicating loosening and precipitate unnecessary revision. A reduction in the incidence of radiolucent lines in cementless UKA may be a mechanism of improving the revision rate.

Radiolucent line adjacent to a cemented UKA.

Theoretically, UKA should be more suitable for cementless fixation than TKA. UKA aims to restore normal ligament tension with minimal implant constraint (to restore the normal ligament-driven kinematics of the native knee). This normal ligament tension is achieved by using either a congruent, unconstrained mobile bearing or a flat, nonconforming fixed-bearing tibial component. The lack of tibiofemoral constraint ensures that (aside from the effects of friction) all the force transfer through the components is compressive, with minimal shear forces. In contrast, with TKA there is excision or release of the cruciate and collateral ligaments, thus requiring a degree of tibiofemoral joint constraint, either by dishing of the tibial plateaux or by the use of a cam-post mechanism. The use of a constrained tibiofemoral articulation leads to the generation of shear forces at the bone-implant interface. If there is eccentric loading in TKA, the tibial component may rock and cause tension and failure at the interface; this contrasts with the situation in UKA, where the forces remain compressive.

Development of cementless UKA

Although UKA was developed in the mid-1970s, there are very few reports of the use of cementless fixation in the first 30 years of its use. Swank and colleagues reported a small series containing both cemented and cementless prostheses in 1993. The cementless prostheses were Fibremesh (Zimmer, Warsaw, IN, USA) and Microloc (Johnson and Johnson, Braintree, MA, USA). The cementless prostheses seemed to perform slightly better than their cemented comparators, although the overall results were poor with a 12% failure rate at 4 years (with a high rate of polyethylene wear).

Although studies of the cementless PCA arthroplasty in the mid-1990s were initially promising, joint registry data demonstrated a very high failure rate, which was attributed to the high degree of tibiofemoral constraint in this design. Similar problems led to the withdrawal of the cementless LCS/Preservation knee (de Puy Johnson and Johnson, Leeds, UK), which had a mobile bearing running in a highly constrained track on the tibial component. Forsythe and colleagues reported encouraging survivorship of 98.2% using the Whiteside Ortholoc II cementless prosthesis (Wright Medical, Arlington, TN, USA). However, these results were presented with very short follow-up (mean follow-up of 40 [12–96] months). Until recent years, the vast majority of UKAs continued to use cemented fixation.

Clinical results of modern cementless UKA

Cementless UKA has become increasingly popular in recent years as more modern implants have been introduced ( Fig. 2 ). Longer term results have been published for 4 cementless UKAs, which are currently in use ( Table 1 ). These cementless UKAs are the Unix (Stryker, Marwah, NJ, USA), the AMC/Uniglide (Corin, Cirencester, UK), the Alpina, and the Oxford (both Biomet, Bridgend, UK).

Graph demonstrating the increasing frequency of cementless UKA in the Australian NJR.

Table 1

Series of cementless UKA

First Author, y	Implant	No.	Mean (and minimum) Follow-up (y)	Survivorship (%)	Functional Outcome	Radiological Outcome
Liddle, 2013	Oxford	1000	3.2 (1)	97.2	1 y OKS 40.2	No loosening. Partial radiolucency in 8.9%
Pandit, 2013	Oxford	30	5 (5)	100	5 y OKS 39.4, KSS(Fcn) 92	No loosening. Partial radiolucency in 7.4%
Hooper, 2012	Oxford	321	2 (2)	99.6	2 y OKS 42.2, HAAS 10.4	1 incompletely seated and loose. Partial radiolucency in 1.5%
Bontemps, 2012	AMC/Uniglide	79	Min 10	97.5	—	—
Saxler, 2004	AMC/Uniglide	89	5.5 (2.3–12.5)	—	KSS(Obj) 93, KSS(Fcn) 89	Radiolucencies in 21.6% (includes cemented knees)
Lecuire, 2008	Alpina	120	6.5	95.7	Combined KSS(O&F) 168.2	Radiolucencies in 4 (1 femoral, 3 tibial; 3.3%)
Hall, 2013	UNIX	85	10 (8–13)	76	—	Tibial lysis in 5%–7%.
Epinette, 2008	UNIX	125	5–13	98.4	KSS(Obj) 97.9, KSS(Fcn) 93.1	Partial tibial radiolucency in 2.3%

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