How does robotic assistance improve implant position in hip and knee replacement?

Introduction

Joint replacement surgery is one of the most consistently successful operations in medicine — but the outcome depends heavily on one thing: where the implant is placed. A hip or knee replacement that is well positioned can function reliably for 20 years or more. One that is placed outside the optimal zone is more likely to wear prematurely, dislocate, or leave the patient with ongoing pain and a leg length that does not feel right.

For decades, surgeons have relied on visual estimation, manual instruments, and experience to position implants during surgery. These techniques work well in straightforward cases — but they are inherently limited by what the human eye and hand can consistently achieve. That limit is where robotic assistance changes the equation.

What does implant position actually mean?

In hip replacement, position refers to the angle and orientation of both the socket (the acetabular cup) and the femoral stem. The cup must sit within a specific range — known as the safe zone — to minimise the risk of dislocation and optimise range of movement. If it tips too far forward or back, the hip is more likely to dislocate, or range of movement is restricted.

In knee replacement, position is even more technically demanding. The implant must replicate the natural alignment of the knee — not just front to back, but rotationally. The cuts made in the bone must be precise to within millimetres. Small errors in rotation or alignment change how forces travel through the knee with every step, accelerating wear on the plastic bearing and affecting how the knee feels and functions.

A hip or knee that is well-positioned at surgery is far more likely to still be performing well a decade later.

What the MAKO robotic system does

The Stryker MAKO system does not operate autonomously. It is a robotic arm that works in partnership with the surgeon — extending precision beyond what the human hand alone can reliably achieve.

A CT scan of the hip or knee is taken pre-operatively. From this scan, the MAKO system creates a detailed three-dimensional model of the patient’s specific anatomy. Using this model, Dr Mooney plans the surgery digitally before making a single cut: the size, position, and exact orientation of the implant are determined in advance, tailored to that patient’s body.

In the operating room, the robotic arm guides bone preparation to match the pre-operative plan precisely. In real time, it provides feedback if the instrument moves outside the planned zone — creating a defined boundary within which the surgeon works.

What the evidence shows

The case for robotic assistance is supported by a growing body of published research. Studies consistently show that robotic-assisted hip and knee replacement achieves more accurate implant positioning compared to conventional manual techniques — with fewer outliers outside the target zone. In hip replacement, this translates to reduced dislocation rates and more consistent leg length outcomes. In knee replacement, improved rotational alignment is associated with better functional outcomes and patient satisfaction scores.

Longer-term data on implant survival is accumulating. The intuitive expectation — that better positioning leads to longer-lasting implants — is biologically sound, and early registry data is beginning to reflect this in reduced revision rates for robotic-assisted cases.

Dr Mooney’s experience with MAKO

Dr Mooney completed a dedicated fellowship in robotic-assisted joint replacement in Perth in 2016 — one of the first surgeons in Australia to undertake such training. Since returning to Adelaide, he has performed hundreds of hip and knee replacements using the MAKO system.

The MAKO system is available at Eastwood Private Hospital, where Dr Mooney operates. Not every orthopaedic surgeon in Adelaide offers robotic-assisted joint replacement — it requires both access to the equipment and specific training in its use.

What this means for patients

For patients considering hip or knee replacement, robotic assistance offers a meaningful advantage — particularly for those who are younger and active, where implant longevity is especially important, or for those with unusual anatomy where standard techniques may be less reliable.

It does not mean the surgery is risk-free or that outcomes are guaranteed. Joint replacement remains a major operation with genuine risks, and recovery takes time regardless of the technique used. But the evidence supports the conclusion that robotic assistance gives the implant the best possible start — and that is what determines how long it lasts.