Why these two techniques are used together

Muscle-sparing surgery sounds straightforwardly better — fewer tendons cut, faster recovery, less disruption. The reasonable concern is whether preserving those tendons forces a trade-off: a narrower operative field could mean less precise implant positioning, and in hip replacement, imprecision carries real consequences.

SPAIRE addresses the soft-tissue side of that equation. By leaving the piriformis and obturator internus tendons completely intact, the approach eliminates the need for the wide posterior exposure that conventional techniques rely on — but that deliberate restraint does restrict the surgeon's direct view of the acetabulum during cup placement.

Mako robotic-arm assistance resolves the geometric side. Using a pre-operative CT scan, the surgeon plans the exact angle, depth, and rotation of the acetabular cup — alongside femoral offset and any leg-length correction — to within approximately 0.5 mm before the first incision is made. Intraoperatively, AccuStop™ haptic boundaries physically prevent the robotic arm from deviating from that plan. The precision, in other words, is established before the restricted exposure becomes relevant.

The pairing is mechanistically logical rather than incidental. SPAIRE provides what no robotic system can supply — biological soft-tissue continuity and immediate posterior stability. Mako provides what a limited visual field cannot — sub-millimetre geometric accuracy independent of the surgeon's direct line of sight. Each technique compensates for the other's inherent constraint.

What SPAIRE actually preserves and what that changes

The short external rotators — piriformis, obturator internus, and obturator externus — act as posterior stabilisers, holding the femoral head in the socket from behind. In a conventional posterior approach, the piriformis and obturator internus are divided to gain exposure; SPAIRE (first described by Kim et al. in 2008 and formally named in a 2017 PubMed-indexed publication) leaves both entirely intact, releasing only the obturator externus, which is subsequently repaired.

That distinction produces two practical consequences.

The first is mechanical. The obturator internus tendon exits the inner pelvis through the lesser sciatic foramen, wraps around the lesser sciatic notch, and passes directly over the posterior surface of the femoral head. Left intact, it functions as a passive sling — described clinically as the 'strap effect' — that resists posterior dislocation from the moment surgery ends, before any healing has occurred. The 2018 Bone & Joint publication confirmed that on-table stability using this approach is sufficient to eliminate all standard post-operative restrictions: patients face no limit on hip flexion beyond 90°, no instruction to avoid crossing their legs, and no prescribed movement ceiling during early recovery.

The second consequence is neurological. The preserved tendons retain their Golgi tendon organs and muscle spindles, which relay continuous positional information to the central nervous system. Severing those mechanoreceptors — as conventional posterior approaches do — removes that feedback loop until slow neural regeneration restores it.

Other routes tackle the exposure problem differently, which is relevant context for understanding where SPAIRE sits. Lateral approaches spare the posterior rotators but divide gluteal tendons instead. The direct anterior approach (DAA) avoids posterior structures entirely, though it requires specialist table equipment and can be more technically demanding in certain patient builds. SuperPATH uses a percutaneous portal technique at the piriformis insertion. Each involves different tissue trade-offs; SPAIRE's specific distinction is posterior-route access with the posterior rotators left undisturbed — a property that shapes both the recovery pattern and the dislocation risk profile, and that neither anterior nor lateral routes replicate.

How Mako robotic-arm assistance closes the accuracy gap

Positioning the acetabular cup correctly demands more than approximate alignment. For the prosthetic socket to function reliably, it must sit within the Lewinnek safe zone — inclination roughly 30–50° and anteversion 5–25° — and millimetre-level deviations in either plane drive wear, impingement, and dislocation risk. Because the cup is fixed permanently in bone, errors cannot be corrected without revision surgery.

Mako (Stryker; in clinical use for hip arthroplasty since 2010) addresses that requirement before the operation begins. A pre-operative CT scan generates a patient-specific three-dimensional model of the individual's anatomy. Using that model, the surgeon maps the exact inclination, anteversion, and depth of the acetabular cup alongside femoral offset and any leg-length correction required, to a planning resolution of approximately 0.5 mm — finer than any intraoperative visual estimate could reliably achieve.

Intraoperatively, AccuStop™ haptic boundary technology converts that digital plan into a physical constraint. The robotic arm produces force-feedback that halts or resists the instrument the moment it approaches the edge of the planned resection zone. Precision is enforced mechanically, not reliant on the surgeon maintaining a clear sightline at every step.

That enforcement is particularly relevant in the context of SPAIRE. The preserved piriformis and obturator internus tendons deliberately narrow the posterior operative field. Mako removes the dependency on that view: because the geometric target is established and machine-enforced before exposure begins, the accuracy of cup placement does not diminish as the visual field does.

A 2024 systematic review (Llombart-Blanco et al., PMC11520809, cited 37 times) found that robotic-arm-assisted THA improves radiological outcomes through enhanced safe prosthesis placement — a result that supports the mechanical rationale for pairing system-enforced accuracy with a muscle-sparing posterior approach.

Why cup position matters this much

Hip dislocation after conventional posterior replacement is not an isolated event with a predictable endpoint. Once it occurs, 57% of patients experience at least one further episode, 11% go on to suffer more than five, and 45.6% of those who dislocate require revision surgery within two years. Revision is a substantially more complex procedure than a primary replacement — longer, technically more demanding, and associated with greater complication rates. The compounding consequence of a single mechanical failure is therefore considerable.

Two distinct failure pathways lead there, and they operate independently.

The first is anatomical: insufficient posterior soft-tissue tension allows the femoral head to migrate out of the socket when the hip is loaded into specific positions. This is the vulnerability window that opens when the posterior tendons are divided, and it takes weeks or months to close as tissue heals.

The second is geometric: even when surrounding soft tissue is intact and functioning, a cup placed outside the clinically established safe zone produces the edge-loading, impingement, or arc-of-movement instability that precipitates dislocation. Soft-tissue health does not correct a structurally off-axis implant.

Recognising that these are separate mechanisms — one biological, one positional — is the practical reason why addressing only the soft-tissue question, or only the implant-positioning question, leaves half the problem unsolved. The dislocation burden above reflects what happens when either half is left unaddressed.

Patient suitability and the assessment process

Neither component of this approach suits every patient, and the case for both depends on individual anatomy that can only be assessed before any decision is reached.

SPAIRE requires a posterior field that can accommodate limited exposure without compromising adequate access to the acetabulum. Patients with a high BMI, significant prior hip surgery, or unusual acetabular morphology may present technical constraints that make a lateral, anterior/DAA, or modified posterior approach more appropriate. The degree and pattern of osteoarthritis also influence approach selection — some joint presentations are better served by greater posterior exposure than SPAIRE's muscle-sparing field provides, and a surgeon trained across techniques will recognise that boundary.

Mako carries its own prerequisites. Pre-operative CT imaging is essential; patients for whom CT is contraindicated, or whose anatomical parameters fall outside the system's planning range, would require a different workflow. The CT dataset must also be of sufficient quality to generate a reliable three-dimensional model — an imaging step completed well ahead of the operative date, not on the day.

Approach selection is a clinical judgement, not a default pathway. Professor Paul Lee, whose practice at Hip Replacement Lincolnshire is built around the SPAIRE framework, assesses each patient individually — weighing anatomy, surgical history, functional goals, and imaging findings to determine whether SPAIRE, a lateral approach, DAA, or another technique is the appropriate fit. Patients can be seen at Sleaford (NG34) or Grantham (NG31) without a GP referral, and there is no NHS-style waiting list for an initial consultation.

Recovery expectations and what the evidence currently shows

The absence of movement restrictions — already established as a direct consequence of on-table stability — shapes recovery in a practical and immediate way. Patients mobilise from the outset without a prohibition list: no 90-degree flexion rule, no leg-crossing ban, no requirement for a raised toilet seat or the contortions that conventional posterior-approach patients must observe for several weeks. How quickly that freedom translates into full function varies considerably between individuals, depending on baseline fitness, pain levels, and pre-operative deconditioning; intact proprioception may support earlier functional confidence, but recovery timelines should be treated as individually variable rather than fixed.

The two evidence strands underpinning this approach are independently solid. The 2018 Bone & Joint publication established SPAIRE's stability profile and the clinical basis for eliminating postoperative restrictions. The 2024 Llombart-Blanco systematic review — cited 37 times as of publication — confirmed that Mako robotic assistance improves radiological outcomes through enhanced safe prosthesis placement. What remains genuinely absent from the published literature is the combined SPAIRE-plus-Mako pathway evaluated as a single long-term endpoint. This is an honest gap rather than a conceptual flaw: the component evidence is strong, the mechanistic rationale for pairing the two is coherent, and prospective data on the combination as a unified workflow is still accumulating. That is a realistic position, not a weakness in the rationale.

Objective gait and biomechanical assessment — for instance, using MAI Motion® pre- and post-operatively — can track how recovery progresses against an individual's own baseline, providing measurable reference points at which the mechanistic argument either confirms itself in the data or prompts timely clinical review.