What these two techniques actually offer

Muscle-sparing hip replacement and a hidden scar — patients sometimes assume these describe the same operation. They do not, and understanding the difference is the first step to a useful conversation with a hip surgeon.

SPAIRE (Save Piriformis And Internus, Repair Externus) is a posterior surgical approach first described in 2016. It addresses what happens deep inside the hip: the small stabilising tendons at the back of the joint are preserved rather than cut and reattached, supporting natural hip stability and reducing the need for post-operative movement restrictions.

The bikini incision operates at a completely different level — the skin. It is a short horizontal cut placed along the natural groin crease so the resulting scar sits beneath a swimsuit or underwear line. It belongs to the direct anterior approach (DAA) family, which accesses the hip from the front.

The two techniques target different things patients often care about: one prioritises structural stability and recovery pace; the other prioritises cosmetic outcome. In selected cases at specialist centres, they can be used together — but each stands on its own merits.

How SPAIRE preserves the hip's natural stabilisers

At the back of the hip joint sits a cluster of short tendons — the piriformis, obturator internus, and the gemelli — which together wrap around the femoral head like a cuff. In a standard posterior hip replacement, several of these tendons are divided to gain access to the joint and then repaired at the end of the operation. Whether that repair fully restores mechanical function is debated; what is not debated is that the division and reattachment take place.

SPAIRE — first described in 2016 and developed at the Exeter Hip Unit — takes a different route. By entering the hip through the interval between the inferior gemellus and quadratus femoris, the surgeon can access the joint while dividing only the obturator externus tendon, leaving the piriformis and conjoint tendons entirely intact. The obturator externus is then repaired at the end of the procedure — the 'Repair Externus' part of the name.

Those preserved tendons matter clinically because they continue to act as a natural tension band across the back of the replaced hip, supporting the new joint in position. In suitable patients, this is associated with a reduced risk of dislocation — and, importantly, the elimination of post-operative hip precautions. The familiar instructions to avoid bending past 90 degrees or crossing the legs, which many patients associate with hip replacement, are not routinely required after SPAIRE.

Published real-world data provide some reassurance about the technique's scale. One surgeon reported using SPAIRE in 1,026 routine primary total hip replacements since February 2016, combining it with MAKO robotic assistance in all cases from 2018 onwards. Definitive comparative evidence, however, is still forthcoming: the NIHR-funded HIPSTER trial — a three-arm randomised controlled trial set against piriformis-sparing and standard posterior approaches — is currently recruiting, and its results are awaited before firm conclusions about relative superiority can be drawn.

Prof Paul Lee trained in the SPAIRE approach under Professor Timperley at the Exeter Hip Unit, the centre where the technique was originally described and refined.

The bikini incision: a skin-level choice for the anterior approach

Langer's lines are the natural tension lines in skin — the directions in which skin stretches and relaxes with movement. An incision that runs along them, rather than across them, tends to heal with a finer, flatter scar. The bikini incision applies this principle directly: it is a short horizontal cut placed along the groin flexion crease, which sits within Langer's lines, so that once healed the scar is concealed beneath swimwear or underwear. The decision concerns only the skin; it does not alter how muscles or tendons are handled at the deeper surgical level.

The practical question is whether the cosmetic gain comes at any functional cost. A 2025 systematic review and meta-analysis pooling nine studies and 2,292 patients found no significant difference in either Harris Hip Score or Oxford Hip Score between the bikini and standard longitudinal DAA incisions — suggesting that the choice of skin incision does not compromise functional recovery. Bikini incision patients did, however, report meaningfully higher scar satisfaction scores. A separate 2025 single-surgeon series of 215 cases documented a 99% stem survival rate and a mean Harris Hip Score improvement from 41.8 before surgery to 92.6 afterwards, including in patients who needed larger acetabular cups of 56 mm or more.

One caution is worth stating plainly: the bikini approach carries a somewhat higher rate of lateral femoral cutaneous nerve (LFCN) neuropraxia — a temporary tingling or numbness along the outer thigh caused by stretch on the nerve during retraction. One series recorded this in 2.3% of cases; the majority of symptoms resolved within six months and did not represent permanent nerve damage.

The technique is not appropriate for all patients. Very obese or very muscular anatomy can make the approach technically demanding, and surgeons generally prefer a conventional posterior incision for complex revision cases where greater surgical access is needed.

How the two techniques differ — and whether they can be combined

The distinction between the two techniques is straightforward once the anatomical layers are separated. SPAIRE is a deep-plane decision — it governs how the surgeon navigates through muscle and tendon at the back of the hip joint. The bikini incision is a skin-plane decision — it governs where the external cut is made at the front. Because they operate at different depths and on opposite sides of the joint, they are not alternatives to each other; they address different patient goals.

SPAIRE prioritises stability and rehabilitation speed by leaving the posterior tendon cuff intact. The bikini incision prioritises scar aesthetics by aligning the cut with natural skin tension lines. Combining the two — a cosmetically placed skin incision paired with a muscle-sparing deep technique — is theoretically possible and is practised at some specialist centres, but it is not standard practice and the choice depends heavily on a surgeon's training, the implant system in use, and individual patient anatomy.

Beyond these two approaches, patients will encounter other options. The standard posterior approach offers familiar visualisation but involves tendon division and reattachment and typically requires post-operative movement restrictions. Lateral approaches carry a recognised risk of abductor muscle damage, which may affect gait recovery. The SuperPATH technique uses a tissue-sparing posterior corridor but requires specific equipment and a learning curve. Each carries its own profile of dislocation risk, nerve proximity, and access for revision surgery.

No single approach is appropriate for everyone. Body habitus, bone geometry, implant requirements, and a surgeon's cumulative experience all feed into the decision — which is why individual assessment by a specialist hip surgeon remains the correct starting point.

Patient suitability: who benefits most from each technique

Choosing between these approaches rests on clinical factors — anatomy, surgical history, body habitus, and a patient's priorities — not on technique preference alone.

SPAIRE: who tends to benefit most

SPAIRE is designed for patients having a primary total hip replacement via the posterior route, where the short external rotator tendons can be preserved without restricting surgical access. Patients with a heightened concern about dislocation — for example those with hypermobility, a history of prior hip surgery, or anatomy associated with instability — may gain particularly from the intact tendon cuff acting as a natural tension band. Those who would find post-operative movement restrictions difficult to manage, for occupational or personal reasons, may also find the unrestricted recovery pathway beneficial.

Where the anatomy demands broader exposure — in complex revision cases, significant deformity, or where implant-size requirements exceed what the tendon-sparing interval can accommodate — a different surgical route is generally more appropriate.

The bikini incision: who tends to benefit most

Candidates most likely to benefit are undergoing primary anterior hip replacement and have a clearly defined natural flexion crease. The cosmetic result depends partly on the incision sitting comfortably within the groin crease, so body habitus and skin quality are relevant factors. For anterior-approach patients who place value on scar concealment, a 2025 meta-analysis of nine studies and 2,292 patients suggests the bikini variant achieves this without compromising functional recovery.

Revision surgery, substantial soft-tissue complexity, or access requirements that exceed what the groin crease can accommodate generally favour a conventional longitudinal skin incision instead.

Selecting the right approach

Approach selection involves weighing imaging findings, bone geometry, implant requirements, medical history, and a patient's individual goals together — a combination that cannot be reduced to a fixed checklist. Prof Paul Lee at Lincolnshire Hip conducts this evaluation before any surgical route is recommended.

What the evidence shows — and what is still being studied

The evidence behind each technique answers a different question. Short-to-medium-term functional equivalence of the bikini incision is well-established; what survivorship data beyond five years will ultimately show remains open. For SPAIRE, a substantial real-world dataset from single-surgeon practice supports the technique's safety profile, but a completed head-to-head RCT against the standard posterior approach is still being gathered by the ongoing NIHR-funded HIPSTER trial.

One gap HIPSTER will not close is a direct comparison between SPAIRE and the bikini-incision anterior approach. Because they belong to entirely different surgical families — posterior versus anterior — no RCT currently pits them against each other on patient-reported outcomes. A patient weighing the two routes cannot resolve that choice by trial data that does not yet exist.

'Awaiting trial data' does not mean either technique is experimental or inadequately supported. Both are practised at established centres with documented outcomes at multiple years of follow-up. HIPSTER will clarify which tendon-sparing posterior strategy offers the clearest patient benefit — it will not alter the principle that approach selection must be driven by individual anatomy, implant requirements, prior surgical history, and the expertise available.

For a patient choosing between a muscle-sparing posterior route and an anterior approach with a cosmetic incision, the practical consequence of these evidence gaps is the same: the decision rests on a specialist assessment of those individual factors, not on a trial result that does not yet exist.

1. [1] A Standard of Care in Hip Arthroplasty: Routine Use of the Tendon-Sparing SPAIRE Technique with MAKO Robotic Assistance. (2024). https://doi.org/10.1302/1358-992x.2024.16.030 https://doi.org/10.1302/1358-992x.2024.16.030
2. [2] The SPAIRE Technique Allows Hip Arthroplasty with Division of Only the Obturator Externus Tendon. (2018).
3. [3] Safety and outcomes of bikini-incision DAA for hip arthroplasty with large acetabular cups (≥56 mm): A single-surgeon series of 215 cases. (2025). https://doi.org/10.1051/sicotj/2025021 https://doi.org/10.1051/sicotj/2025021
4. [4] Longitudinal incision vs bikini incision for anterior total hip arthroplasty: A systematic review and meta-analysis. (2025). https://doi.org/10.52628/91.3.14375 https://doi.org/10.52628/91.3.14375
5. [5] For many but not for all: the bikini incision direct anterior approach for total hip arthroplasty. A narrative review. (2024). https://doi.org/10.1186/s10195-024-00812-z https://doi.org/10.1186/s10195-024-00812-z