What makes someone a good candidate for SPAIRE

For most patients considering hip replacement, the first question is not which surgical approach but whether surgery is warranted at all. Once that decision is made, SPAIRE — the muscle-sparing modified posterior approach developed by Kim et al. in 2008 and refined at the Exeter Hip Unit — is designed to suit the full standard range of primary hip arthroplasty indications: osteoarthritis, rheumatoid arthritis, and osteonecrosis of the femoral head.

Beyond elective replacement, the technique has been applied successfully in bipolar hemiarthroplasty for femoral neck fractures in geriatric patients, where early stability and rapid functional recovery are especially critical.

Certain patient profiles stand to gain the most from soft-tissue preservation. Multivariate data shows that dislocation risk after traditional posterior total hip arthroplasty rises sharply in patients with a BMI below 20, those under 65, and women — groups in whom the intact 'strap effect' of the preserved obturator internus tendon addresses the precise vulnerability created by a conventional approach. High-activity individuals who want to avoid the movement restrictions that accompany standard posterior surgery are in a similar position.

None of these factors operates as a simple threshold. The profiles above indicate who is likely to benefit most — not who automatically qualifies.

The anatomy behind how SPAIRE reduces dislocation risk

Standard posterior hip replacement divides the piriformis and obturator internus tendons to reach the joint. SPAIRE leaves both intact — and the anatomy of the obturator internus explains why that decision changes a patient's dislocation risk from the first moment they stand up.

The obturator internus exits the pelvis through the lesser sciatic foramen, loops around the lesser sciatic notch, and runs directly across the posterior surface of the femoral head before inserting on the greater trochanter. With the tendon preserved, this course creates the 'strap effect': a passive biological tether that presses against the femoral head and resists backward displacement whenever the hip moves into positions traditionally considered dangerous. The tether is in place from the moment the wound is closed — there is no healing lag.

The second mechanism operates at a neurological level. Golgi tendon organs and muscle spindles within the intact short external rotators continuously relay positional information to the nervous system, allowing reflex muscle responses that guard against subluxation. Traditional posterior approaches sever these mechanoreceptors; until the tendons regenerate — if they do — the joint lacks that real-time sensory feedback. The result is a period of functional 'neurological blind spot' that SPAIRE avoids entirely.

It is the combination of these two effects — a mechanical tether active from day one, and uninterrupted proprioceptive signalling — that underpins the removal of traditional hip precautions after SPAIRE, and supports earlier, more confident movement in the recovery period.

How BMI and body composition affect suitability

Body weight tends to be one of the first things patients ask about when considering hip replacement — and for SPAIRE, the answer runs counter to the usual assumption that higher weight is the main obstacle.

At the lower end of the BMI scale, patients with a BMI below 20 carry reduced periarticular soft tissue around the hip joint, which diminishes the passive muscular resistance that would otherwise guard against dislocation after traditional posterior surgery. SPAIRE directly compensates for this deficit: the preserved obturator internus tendon maintains its anatomical 'strap' across the posterior femoral head, replacing the lost passive resistance with an intact biological tether from the moment of wound closure.

At the higher end, no upper BMI exclusion threshold for SPAIRE has been established in the available clinical literature. Because the approach uses a posterolateral corridor, it does not require the anterior tissue mobilisation that makes the Direct Anterior Approach progressively more demanding in heavier or heavily muscled patients — a technical distinction that means SPAIRE's applicability is not narrowed by the same anatomical challenge that can limit DAA use in this group.

Where BMI does become clinically relevant at higher levels, the concern relates to general anaesthetic and systemic surgical risk — a consideration that applies equally across all hip arthroplasty approaches, not to SPAIRE specifically. Determining where an individual patient sits within that risk framework requires surgeon-led pre-operative assessment rather than a fixed weight threshold.

Prior surgery, scarring, and anatomical complexity

Patients who have had previous surgery around the hip — particularly via a posterior or posterolateral incision — raise a question the current published literature does not yet fully answer. No formal SPAIRE-specific guidance sets rules around prior posterior hip procedures, and that gap is worth naming plainly.

The clinical inference is nonetheless sound. SPAIRE's principal benefits — the obturator internus strap effect and intact proprioceptive signalling — depend on posterior soft tissues that are anatomically whole. Where a previous posterior approach, trauma, or significant scarring has altered or damaged those structures, the mechanical and neurological advantages the technique is designed to deliver may be reduced. In such cases, a surgeon may instead favour a traditional extensible posterior approach, which provides broader access to modified anatomy, or consider an anterior corridor that avoids the scarred tissue altogether.

Prior surgery through an anterior incision, or unrelated lower-limb procedures, does not carry the same theoretical implications for the strap-effect mechanism — the posterior soft-tissue architecture remains undisturbed.

None of this constitutes a blanket contraindication. The extent of any previous soft-tissue disruption, how well the tissue has healed, and the degree of anatomical change all vary considerably between patients. A complex surgical history makes individual surgeon assessment more important, not SPAIRE categorically unsuitable.

Cases where a different surgical approach may be preferable

Choosing SPAIRE over another approach is not automatic — and for certain clinical situations, a different access route is the more appropriate choice.

The traditional extensible posterior approach retains one clear advantage: breadth of access. In revision arthroplasty, where the surgeon must navigate around implanted components, cement, or fibrous scar tissue, or in primary cases involving gross deformity or severe bone loss, the wider surgical corridor that the traditional posterior approach provides may simply outweigh the benefits of soft-tissue preservation. Extensibility is not a luxury in these settings — it is a clinical requirement.

The Direct Anterior Approach (DAA) presents a different set of tradeoffs. In straightforward primary cases with standard anatomy, DAA can deliver faster early mobilisation and lower short-term pain scores. Against that, published data associates DAA with longer operative times, higher intraoperative blood loss, lateral femoral cutaneous nerve (LFCN) neurapraxia causing lateral thigh numbness, and — during the learning curve — an elevated risk of intraoperative femoral fracture. It is not categorically inferior, but it is not categorically superior either.

One concern sometimes raised about muscle-sparing approaches — that smaller exposure compromises implant placement accuracy — is addressed in SPAIRE practice by Mako robotic-arm assistance, which provides sub-millimetre positioning guidance and offsets the more limited visual field.

Long-term implant survival rates are equivalent across modern approaches, so the decision rarely turns on prosthesis longevity. The practical mapping is more direct: revision or complex deformity favours traditional posterior access; straightforward anatomy with an early-return priority may make DAA competitive; dislocation-risk profiles in primary cases are where SPAIRE's soft-tissue preservation confers its greatest advantage. A surgeon-led assessment determines which profile the individual patient actually fits.

Getting a suitability assessment

Suitability for SPAIRE is not a binary determination that can be read from clinical profiles alone. Hip anatomy varies, surgical histories differ, and the interplay between soft-tissue integrity, activity goals, and medical background requires direct clinical examination to interpret reliably.

Assessments at Hip Replacement Lincolnshire are informed by the specialist posterior technique developed by Professor Paul Lee. Consultations take place at clinic sites in Sleaford (NG34) and Grantham (NG31), Lincolnshire, and no GP referral is required. Patients travelling from outside the county are also regularly seen, and there is no NHS-style waiting list.

A clinical review will typically examine hip movement, imaging, the extent of any soft-tissue or bony change, and the patient's recovery priorities — the same factors that determine whether SPAIRE, a different surgical approach, or a deferred decision is most appropriate for that individual.

To arrange an assessment, visit hipreplacementlincolnshire.co.uk.