Dr Christopher Jones, Orthopaedic Surgeon, Perth

For 15 years, the aim of computer navigation has been to improve the precision and accuracy of component positioning in total knee replacement (TKR). In the UK and the USA only 3-5% of TKRs per year use navigation as part of computer assisted surgery (CAS) whereas in Australia, CAS navigation has increased to 30.8% in 2016 (from 2.4% in 2003).

ED: The jury is still out on computer use in TKR. Can we afford it and with our ageing patients will it lessen revisions?

Given the similarities between the three countries in many aspects of surgical practice, the reasons for this difference are hard to fully elucidate. While current evidence demonstrates that computer navigation improves component alignment in TKR, apart from one subgroup of patients <65years (see below), there is no demonstrated clinical superiority of CAS in terms of improved reported outcomes and/or decreased revision rates in the long term.

Without CAS – what is done

Historically, TKR aims to restore the mechanical alignment of the lower limb by aligning the components perpendicular to the mechanical axis of the femur and tibia. Using manual instrumentation, the tibia has a guide set 90 degrees to the mechanical axis while the femur is prepared with an anatomic valgus cut (typically 5-7degrees).

Figure 1. A. Alignment goals in total knee replacement B. Computer navigated TKA

In this way, the overall mechanical alignment of the limb is neutral +/- 3 degrees and the line of force is transmitted from the centre of the hip and the middle third of the knee, to the centre of the talus (Figure 1). Deviation of the mechanical axis away from neutral alignment results in abnormal forces. As well, most TKR components are designed so the bearing forces are transmitted perpendicular to the bearing surface.

Optimal alignment for TKR has been debated hotly in the orthopaedic literature, with the neutral mechanical axis challenged by proponents of “kinematic alignment”. Despite this debate, the importance of post-operative alignment goal, in terms of TKR longevity, has been clearly established.

Alignment using computer navigation is more reliable and reproducible than manual guides and multiple systems now exist with rapid advancements in computing power stimulating the next stage in CAS, robotics.

Is CAS better or not?

Figure 2. CAS for TKR – one such system

Interestingly, while neutral alignment independently improves TKR survivorship and computer navigation independently improves the accuracy and precision of alignment; the logical conclusion that CAS navigation will improve TKR survivorship, remains definitely unproven.

Tantalisingly, perhaps one study does. Using data from the Australian Orthopaedic Association National Joint Replacement Registry, De Steiger et al (2015) compared revision rates of 44,473 CAS navigated TKR compared to 270,545 conventional TKR, over 9-years post-operative follow-up.

For the sub-group of patients < 65 years, there was a significant decrease in the revision rate in the computer navigated group (6.3%, 95% CI = 5.5 to 7.3) compared to the conventional TKR group (7.8%, 95% CI = 7.5 to 8.2). The authors concluded that CAS navigation led to a significant reduction in the leading cause of revision in Australia (aseptic loosening) with a hazard ratio of 1.38 (95% CI = 1.13 to 1.67, p = 0.001) in that patient sub-group.

Looking ahead

As Friedman et al said (2017), the value of new technology (benefit divided by the cost) is an increasingly important issue in medicine, and in orthopaedic surgery in particular. There is an increased initial cost of using computer navigation for TKR, estimated by Novak et al (2007) at up to $1500 per case, however the potential cost saving of reduced revisions if improved alignment is proven to increase survivorship needs to be considered.

References available on request

Key Messages

  • Computer navigation for TKR is used more in Australia (30%) than the USA or the UK (3-5%).
  • Precise alignment goals for TKR are still being debated.
  • Computer navigation achieves more accurate alignment in TKR.
  • Some patients <65 years have a lower revision rate for computer navigated TKR compared to conventional TKR
  • Otherwise, no overall benefit in patient outcomes for computer navigated TKR has been proven
  • There is increased cost and time associated with computer navigation for TKR

Author competing interests: nil relevant.

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