Relevant background

Contemporary TKA designs vary in the degree to which the tibial and femoral articular surfaces are constrained to one another by patient soft tissues versus elements of the prosthesis itself. It is generally accepted that lower‐constraint designs are preferable in the context of modest deformity and competent knee ligaments, allowing forces across the knee joint to be maximally absorbed by patients’ own soft tissues. In contrast, greater amounts of implant constraint typically require more bone resection may constrain joint motion and result in greater forces on implant components as well as the bone–implant interface, increasing the risk of earlier failure.

The large majority of primary TKA procedures involve the implantation of lower‐constraint implants that rely entirely on the medial and lateral collateral ligaments for varus/valgus stability, while sacrificing the anterior cruciate.

However, primary TKA implant designs differ in terms of treatment of the posterior cruciate ligament (PCL). Generally speaking these can be divided into two groups: those that retain the PCL (cruciate‐retaining, or CR) and those that sacrifice it (PCL‐sacrificing or posterior stabilized, or PS). CR designs have limited conformity between the tibial and femoral articulations, conceptually relying on the retained PCL to limit posterior tibial translation and facilitate femoral rollback in flexion. In contrast, PS designs rely on the tibiofemoral articulations for femoral rollback. Traditionally, this has been achieved through the use of a cam‐and‐post mechanism, whereby a post on the tibial polyethylene engages with the femoral component as the knee is flexed.

Advocates of CR implants have suggested advantages to their use, including less femoral bone resection, lower risk of iatrogenic fracture, more physiologic knee kinematics, superior function owing to retention of proprioceptive receptors within the PCL, and lower rates of polyethylene liner failure particularly as compared to the cam‐and‐post design. In contrast, advocates of PS implants suggest benefits, such as greater range of motion (ROM), more predictable outcomes, and lower risk of late instability attributable to PCL rupture.

Importance of the problem

TKA is among the most common contemporary surgical procedures. In 2012, 700 100 knee arthroplasty procedures were performed in the United States, making it the most common operating room procedure with a population rate of over wo surgeries per 1000 people.¹ In addition to being common, surgical treatment of knee osteoarthritis represents a considerable healthcare cost burden. In 2013, osteoarthritis was the second‐most‐expensive condition billed to Medicare, and the most expensive condition billed to private insurance in the United States.² Although TKA has shown good results in decreasing pain and improving function in patients with symptomatic degenerative disease, up to 20% of patients remain dissatisfied with the results of their surgery.³ Considering the marked health economic burden of TKA, and the notable patient dissatisfaction rate with this procedure, it is critical that evidence‐based decisions be made around implant selection so as to maximize healthcare value.

Rationale and clinical comment

TKA is an elective procedure, with the primary therapeutic goals of improving patients’ function and quality of life. Consequently, assessment of the outcomes of this procedure should be made from the patient’s perspective. Thus, it is important to consider whether the choice of CR versus PS implants results in any difference in outcomes of the surgery from the patient perspective, as measured using validated patient‐reported outcome measures.

Available literature and quality of the evidence

Multiple randomized controlled trials (RCTs) are available assessing this question. Four of these were summarized in a Cochrane review published in 2013.⁴–⁸ A literature search of Embase and Medline databases identified an additional seven subsequently reported RCTs.⁹–¹⁵ Thus, a total of 11 randomized trials were used to address this question, all with a level of evidence of I. Given this available evidence, no lower‐quality studies were used to address this question.

Findings

A Cochrane review that included studies published up to December 2012 found no significant difference in patient‐reported outcome scores between CR and PS TKA across four studies that used the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).¹³–¹⁶ One more recent study was identified that reported WOMAC scores, which found a mean difference in WOMAC scores of 3.8 points (significant difference [SD] 2.8 points; p <0.001) favoring an ultra‐congruent PS liner over a CR implant in 210 knees. Pooled results from these studies encompassing 641 knees at follow‐up times from 24 to 87 months found an absolute difference in scores of 2.11 points in favor of PS knees (95% confidence interval [CI]: −0.11–4.33 points),¹² which narrowly missed statistical significance and is unlikely to reach the threshold of a minimal clinically important difference (MCID). The overall quality of evidence was low, with a frequently unclear or high risk of bias.

Four studies were identified that reported Visual Analog Scale scores for knee pain,¹³,¹⁴,¹⁷,¹⁸ with no significant differences identified. Pooled results encompassing 315 knees at follow‐up times ranging from one to six years found no significant difference in outcomes (1.44 mm on a 100 mm scale in favor of PS; 95% CI: −1.8–4.68 mm).

Of the remaining four studies, three reported individual WOMAC domain scores only,⁹,¹⁰,¹⁵ while one reported individual domain Knee Osteoarthritis Outcome Scores (KOOS) only.¹¹ Of these, one study identified worse WOMAC pain subdomain scores in those patients who received a PS design (mean 4.2 points vs 2.5 points; p = 0.043), but no difference in other domains.¹⁵ The remaining studies did not identify any significant differences in any PROMs.

Overall, the presently available evidence is limited somewhat by relatively small individual studies and variability in both outcome measures and follow‐up intervals. Nevertheless, it is possible to say with moderate certainty that there is no clinically relevant difference in patient‐reported outcomes of TKA associated with the use of CR as compared to PS designs.

Resolution of clinical scenario

In terms of patient‐reported outcomes of TKA, the best available evidence suggests no difference in outcomes associated with the use of CR versus PS designs. In the absence of other factors, and assuming the surgeon is comfortable with both designs, either would be an excellent choice for this patient.

Rationale and clinical comment

Related posts:

The Direct Anterior Approach Patient‐Specific Instrumentation in Total Knee Arthroplasty Management of Glenoid Bone Loss Distal Femur Fractures Periprosthetic Fractures: Knee Lisfranc Injuries

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