Why surgeons use robotics and muscle-sparing surgery together

When hip replacement is being considered, a reasonable question is what actually separates one surgical approach from another — and whether the differences matter enough to affect recovery.

The honest answer is that hip replacement has two distinct technical problems, and most approaches only solve one of them.

The first is soft-tissue disruption. Standard posterior techniques sever the tendons surrounding the hip to gain access to the joint. Cutting those tendons removes the biological structures that hold the hip in place and relay positional feedback to the nervous system — leaving the joint mechanically and neurologically vulnerable while tissues heal. That healing window, typically around 90 days, is precisely when dislocation risk is highest.

The second is component positioning. Even when soft tissue is handled well, placing the acetabular cup and femoral stem at the right angle, depth, and offset is technically demanding. Errors — even small ones — cause impingement, leg-length discrepancy, and accelerated wear over time.

The SPAIRE approach addresses the first problem by preserving the major posterior tendons entirely. Mako robotic assistance addresses the second by planning implant placement to within approximately 0.5 mm before surgery and physically enforcing those targets intraoperatively. Crucially, SPAIRE's muscle-sparing exposure narrows the surgeon's direct view of the joint — which makes robotic precision more useful, not less. Together, the two techniques target different failure modes simultaneously, supporting faster recovery pathways and, in selected patients, same-day or next-day discharge.

What the SPAIRE technique preserves — and why it reduces dislocation risk

SPAIRE is a modified posterior approach — it uses the same general access route as standard posterior hip replacement, but what happens to the surrounding tendons is categorically different. During a conventional posterior operation, the piriformis, obturator internus, and gemelli tendons are divided to reach the joint. In SPAIRE, those structures are left entirely undisturbed. Only the obturator externus tendon is released and subsequently repaired — hence the acronym: Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus.

Leaving those tendons intact creates two distinct stabilising effects.

The first is mechanical. The obturator internus tendon passes directly over the posterior aspect of the prosthetic femoral head at approximately the joint's centre of rotation, acting as a biological tether that resists the femoral head pulling backwards out of the socket. This 'strap effect' operates through both active muscle contraction and the passive viscoelastic resistance of the tendon itself.

The second is neurological. The Golgi tendon organs and muscle spindles within the preserved short external rotators continue sending real-time positional signals to the central nervous system immediately after surgery. Conventional tendon division interrupts those signals until healing is complete — a period during which the joint cannot accurately regulate its own position.

Both effects carry a clinical consequence. Writing in Bone & Joint (2018), Timperley confirmed that on-table stability following SPAIRE is sufficient that no post-operative movement restrictions are required — a material contrast with standard posterior techniques. The same paper noted that SPAIRE damages fewer tendons than any other hip approach, including direct anterior.

The dislocation statistics explain the stakes: once a hip dislocates after replacement, 57% of patients experience a further event and 45.6% require revision surgery within two years. SPAIRE does not eliminate that risk, but by preserving intact tendon mechanics from the moment surgery ends, evidence suggests it may substantially reduce early post-operative exposure.

What Mako robotics contributes: planned precision where vision is restricted

Robotic assistance in hip arthroplasty enters the picture at two distinct stages — before the patient arrives in theatre and during the operation itself — and both are clinically significant.

The pre-operative stage begins with a CT scan. Mako (Stryker's robotic-arm system) uses the imaging data to build a patient-specific three-dimensional model of that individual's hip anatomy. From this model, the surgeon defines the exact angle, depth, and rotation of the acetabular cup, the femoral offset, and any leg-length correction — to within approximately 0.5 mm. Every target is confirmed and locked before the first incision is made.

During surgery, the robotic arm enforces those targets through haptic boundaries: physical force-feedback that prevents the instrument from removing bone outside the pre-mapped resection zone. This is a mandatory mechanical constraint, not a display warning or advisory prompt. If the instrument approaches the edge of the planned area, the arm physically resists. Component malpositioning is the most common biomechanical failure mode in total hip arthroplasty, producing impingement, dislocation, accelerated bearing-surface wear, and leg-length discrepancy. A study of 534 Mako-assisted hip replacements (Perazzini, PMC, 2020) found that robotic-arm planning substantially improves implant placement accuracy across each of these parameters.

Where this precision becomes especially relevant is in the context of SPAIRE's muscle-sparing exposure. Preserving the posterior tendons intact reduces the direct visual field available during bone preparation and cup seating — a genuine technical trade-off. Mako addresses it directly: the surgeon sees less of the joint surface, but the component is placed to tighter tolerances than freehand surgery allows. The plan executes what vision cannot fully confirm.

Mako is available in a limited number of advanced hip arthroplasty centres rather than universally, which is a practical consideration for patients deciding where to seek assessment and treatment.

How the two approaches work together: dual-layer stability

During surgery, the SPAIRE exposure contributes something the robotic plan alone cannot provide: direct tactile feedback. Because the posterior tendons remain intact, their physiological tension is preserved — and the surgeon can judge the effect of any leg-length or offset adjustment in real time. That live biological signal cross-checks the Mako plan rather than duplicating it; the two sources operate by different mechanisms and confirm each other independently.

This intraoperative cross-check is one expression of a broader dual-layer structure.

Layer one: biological stability

The intact obturator internus tendon exerts what is clinically described as the 'strap effect' — a continuous tether across the posterior aspect of the prosthetic femoral head that resists dislocation through both active contraction and passive tendon tension. Alongside this, the Golgi tendon organs and muscle spindles within the preserved short external rotators remain functional from the moment surgery ends, providing uninterrupted proprioceptive feedback. Conventional tendon division eliminates both effects until healing occurs.

Layer two: mechanical precision

Mako's pre-operative CT plan defines the acetabular cup angle, depth, and rotation to within approximately 0.5 mm, targeting the Lewinnek safe zone — the geometric envelope associated with the lowest dislocation and impingement risk. Haptic boundaries enforce that plan intraoperatively, physically preventing bone removal outside the pre-mapped zone. If the biological layer is transiently compromised — by post-operative swelling that reduces tendon tension, for instance — the implant geometry continues to protect the joint independently.

Professor Timperley, who developed SPAIRE at the Exeter Hip Unit, has formalised this combination in a dedicated SPAIRE/Mako professional video series covering each operative stage from pelvic pin placement to socket implantation with robotic assistance. Professor Paul Lee, trained under Timperley, integrates both techniques as his routine approach — a clinical practice reflecting deliberate design, not incidental co-use.

Patient suitability: who benefits from SPAIRE with robotic assistance

Not every patient who is a candidate for hip replacement will be suitable for the SPAIRE technique, and the same is true of robotic assistance — both require individual clinical appraisal before a decision is made.

SPAIRE is a posterior approach, and certain anatomical factors influence its feasibility. Prior posterior surgery, specific hip morphologies, and high BMI — which affects soft-tissue depth and operative exposure — can all affect whether the approach is practical for a given patient. This is worth weighing alongside alternatives. The direct anterior approach (DAA) similarly avoids major muscle division and is widely used; it carries a different nerve-risk profile (particularly the lateral femoral cutaneous nerve) and typically requires specialist positioning equipment. The standard posterior approach preserves a wider operative field and remains the most broadly available technique, but involves tendon division and a period of post-operative movement restrictions that SPAIRE does not. Each trade-off is real, and approach selection is guided by anatomy, surgeon training, and available facilities rather than a single hierarchy of superiority.

Mako adds a further step: robotic-arm assistance requires a pre-operative CT scan, and robotic candidacy is assessed alongside general hip arthroplasty workup. Centres offering both SPAIRE and Mako remain relatively few.

Rapid recovery pathways — same-day or next-day discharge in selected cases — are achievable when SPAIRE and Mako are combined, but eligibility depends on medical comorbidities, home circumstances, and access to physiotherapy. These are individual determinations made at assessment, not default outcomes of the technique.

The British Hip Society (britishhipsociety.com) publishes independent patient information on hip replacement options, including guidance on questions to raise with a surgeon before committing to an approach. Where specialist assessment is the next step, Hip Replacement Lincolnshire — part of the MSK Doctors group — accepts patients without a GP referral.

What the evidence shows — and where honest gaps remain

Three categories of evidence carry different weights, and distinguishing them gives a clearer picture of where the combined approach stands.

The structural and mechanical case is well-founded. SPAIRE's tendon-preservation mechanism and the on-table stability it produces have been formally reported in peer-reviewed literature; Mako's accuracy advantage in component positioning is supported by prospective series. These findings underpin the rationale described in earlier sections and do not need restating here.

The proprioceptive continuity argument is a different matter. The claim that intact Golgi tendon organs and muscle spindles within the preserved short external rotators maintain real-time neurological feedback from the moment surgery ends is anatomically coherent and mechanistically expected — but has not yet been formally confirmed in prospective outcome data. The literature describes it as 'likely' rather than proven. It is a plausible mechanism worth understanding, not an established clinical effect.

The combined SPAIRE-plus-Mako picture carries a third gap: randomised data directly comparing the combination against either technique alone — on dislocation rates and patient-reported outcomes — remains sparse. The 2018 Bone & Joint paper noted ongoing randomised controlled trials at the time of publication. No long-term survivorship data specific to the combined approach is yet available; the approximately 58% 25-year survival figure for total hip arthroplasty reflects the broader evidence base across approaches, not this combination specifically.

The clinical rationale for using both techniques together is mechanistically coherent, grounded in well-characterised mechanisms, and supported by early evidence — a reasonable basis for consultant-led decision-making, with prospective trials continuing to add detail.