The visual-field problem SPAIRE creates — and why it matters

Every total hip replacement involves placing a prosthetic socket — the acetabular cup — at a precise angle within the pelvis. Get that angle wrong by even a few degrees and the risk of dislocation rises sharply; the bearing surfaces wear unevenly; and a revision procedure becomes more likely. Given that roughly 58% of hip replacements are estimated to last 25 years, the accuracy of that initial placement carries consequences measured in decades.

In a conventional posterior approach, the surgeon detaches the piriformis and obturator internus tendons from the bone during the operation. Releasing those tendons widens the operative corridor and gives a direct line of sight to the acetabular rim — the reference points the surgeon uses to judge cup inclination and anteversion as the implant is seated.

SPAIRE — which stands for Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus — deliberately keeps those tendons intact, passing between them rather than cutting through them. That decision preserves the posterior soft-tissue envelope and protects the mechanoreceptors within it, but it also narrows the visual corridor. The surgeon cannot rely on the same direct anatomical sightlines available in a conventional posterior exposure.

This is not a design flaw. It is a deliberate engineering trade-off: soft-tissue continuity is prioritised over visual convenience, on the condition that positional accuracy is recovered by another means.

How Mako robotic guidance fills the visual gap

Mako's answer to that trade-off begins before the patient enters the operating theatre. Using a pre-operative CT scan, the system builds a three-dimensional model of each patient's pelvis, and the surgical team uses that model to define the target acetabular inclination, anteversion, and cup depth in advance. By the time the first incision is made, the positional targets are already computed and locked — derived from anatomy, not estimated from what the surgeon can see on the day.

Intraoperatively, the robotic arm enforces a haptic boundary: physical resistance that prevents the reamer or cup impactor from straying outside the pre-planned zone. The surgeon still drives the instrument, but the robot constrains its envelope of movement. Accuracy, sub-millimetre in principle, flows from the CT plan and haptic constraint rather than from any line-of-sight view of the acetabular rim.

The practical consequence for SPAIRE is direct. A GPS navigation system does not require the driver to see the full road ahead — it already holds a precise map and corrects for deviations in real time. Mako operates on the same logic: because the robot knows where the cup boundary sits, the narrowed visual field created by the intact piriformis and obturator internus tendons does not degrade positional accuracy. What was a potential limitation of the muscle-sparing exposure becomes, in mechanical terms, irrelevant to the placement outcome. This is the operating principle on which the combination rests — one supported by the design of the technology itself rather than by published head-to-head trials comparing this pairing specifically, which have not yet emerged.

Why cup positioning determines long-term hip replacement outcomes

Safe-zone numbers give the stakes a concrete frame. The acetabular cup should sit at roughly 40° of inclination and approximately 15° of anteversion; stray outside that corridor and the joint risks impingement on one side, edge-loading on the bearing surface, or posterior instability on the other. What those consequences mean in practice was established earlier — what matters here is that the safe zone is not the same for every patient.

Standard manual technique has historically aimed at population-average targets: angles derived from large cohort data rather than from any individual's pelvis. That approach works adequately in straightforward anatomy, but it leaves patients with structural variation — acetabular dysplasia, meaningful leg-length inequality, or a hip altered by previous surgery — more exposed to placement error. Their safe zones may be shifted, tilted, or narrowed relative to the textbook norm, and a target calibrated to the average may miss the individual.

Mako's pre-operative CT model defines targets from each patient's own three-dimensional pelvic geometry. The plan is drawn on their anatomy, not borrowed from a population mean. For SPAIRE patients, that personalisation matters on two levels: robotic precision addresses the one technical vulnerability of the muscle-sparing exposure — restricted visual access to the acetabulum — while the preserved posterior soft tissue addresses the recovery and stability vulnerabilities that have historically been the drawback of the standard posterior approach.

What the preserved tendons add beyond the robot's precision

Robotic precision positions the cup correctly; the intact tendons then work to keep it that way. These are complementary functions, not redundant ones.

The key mechanism is the obturator internus tendon, which courses laterally from the lesser sciatic foramen and passes directly over the posterior femoral head at or near the true centre of rotation. Because SPAIRE leaves this tendon undisturbed, it continues to act as a biological seatbelt across the back of the joint — what clinicians describe as the 'strap effect'. Through both active muscular contraction and passive viscoelastic tension, it creates a constant tether that resists posterior subluxation. A precisely positioned cup reduces the geometric opportunity for dislocation; an intact obturator internus actively opposes the forces that would cause it. The two defences operate in parallel, not in sequence.

The neurological contribution is equally significant. Golgi tendon organs and muscle spindles embedded in the preserved short external rotators continue providing afferent feedback to the central nervous system from the first day after surgery. The joint can, in effect, sense its own position. When those mechanoreceptors are severed in a conventional posterior approach, this proprioceptive continuity is lost until slow neural regeneration occurs — a period during which the hip is, in physiological terms, neurologically blind to extremes of range. The preserved rotators in SPAIRE are thought to contribute to lower early dislocation risk and more confident early mobilisation, though this remains a mechanism-based argument rather than a finding from a dedicated randomised trial of the SPAIRE–Mako pairing specifically.

Finally, the retained physiological tension across the posterior capsule and tendons gives the operating surgeon direct tactile information about leg length and femoral offset during the procedure — a real-time biological signal that runs alongside, rather than competing with, the robot's positional data.

Patient suitability: who benefits from this combination

Deciding between surgical approaches starts with anatomy, not preference. Mako-assisted SPAIRE tends to suit patients with well-preserved bone stock, no significant acetabular dysplasia, and a clinical priority of rapid soft-tissue recovery — those for whom early proprioception and minimal tendon disruption are meaningful gains. For this group, the robotic plan converts the approach's limited visual field from a practical risk into a non-issue.

Not every patient fits that profile, and it is important to say so directly. Significant acetabular dysplasia, severe hip deformity, revision of a previous hip replacement, or morbid obesity can each make the SPAIRE exposure insufficiently wide even with robotic guidance — the anatomy demands more access than the muscle-sparing interval allows. In those cases, a wider opening may be the safer clinical decision regardless of the robotic platform available.

The alternative approaches each carry their own honest tradeoffs:

• Standard posterior THA with Mako offers a fuller acetabular view by releasing and repairing the posterior tendons, which suits complex anatomy — but sacrifices the proprioceptive continuity and strap-effect stability that SPAIRE retains.
• Anterior/DAA avoids the posterior capsule entirely and allows imaging confirmation of leg length intraoperatively, but carries a risk to the lateral cutaneous nerve of the thigh and tends to be technically more demanding on revision.
• Lateral approaches protect posterior stability structures by design but involve splitting the gluteal muscle, which creates a different rehabilitation challenge and a recognised risk of abductor weakness.
• SuperPATH is a posterior mini-approach with a different rotator interval; it preserves some posterior tissue but offers less consistent proprioceptive benefit than SPAIRE in published descriptions.

In plain terms: SPAIRE plus Mako offers the most when the patient's hip anatomy is amenable to a limited posterior opening and the recovery goal is early, confident mobilisation. It offers less when the anatomy itself demands wider exposure. A specialist assessment — weighing CT geometry, bone stock, and revision history — is the only reliable way to determine which side of that line a given patient falls on.

Getting a specialist assessment for Mako-assisted SPAIRE

The pairing of SPAIRE and Mako reflects a broader shift in hip arthroplasty: the long-held tension between soft-tissue preservation and positional accuracy is beginning to dissolve. For most of THA's history, surgeons faced a genuine trade-off — widen the exposure and see clearly, or spare the muscles and accept greater positional uncertainty. Computational pre-operative planning changes that calculus. When inclination, anteversion, and cup depth are defined before the first incision, the surgeon's visual field during implantation becomes less decisive. What remains open is longer-term comparative data; controlled trials measuring cup-positioning accuracy in Mako-assisted SPAIRE against Mako with a standard posterior approach have not yet been published. The mechanical argument is well-reasoned; the trial evidence specific to this pairing is still accumulating.

For patients considering this route, the appropriate first step is a consultant-led assessment — a review of individual anatomy, bone stock, and recovery priorities — not a commitment to a specific technique. Professor Paul Lee, who trained in SPAIRE at the Exeter Hip Unit under Professor Timperley, integrates this approach with a Rapid Biological Recovery programme and sees patients at the Hip Replacement Lincolnshire clinics in Sleaford (NG34) and Grantham (NG31), both part of the MSK Doctors group. No referral is required; imaging can be arranged at the Sleaford site, where an Open MRI scanner is available. To book an initial assessment, visit hipreplacementlincolnshire.co.uk.