What each approach does to the hip joint

When a surgeon plans a hip arthroplasty, the approach they choose determines which structures are disturbed — and which are left entirely intact.

The hip is a ball-and-socket joint wrapped by seventeen muscles arranged into four functional groups: gluteal, lateral rotator, adductor, and iliopsoas. Reaching the joint requires working either through or around this muscular envelope, and where the incision travels makes a fundamental difference to what the body must repair afterwards.

SPAIRE — formally named in 2016 as Saving Piriformis And Internus, Repair Externus — is a modified posterior approach that enters the hip in the interval below the piriformis tendon. Only the obturator externus tendon is divided, and it is repaired at closure; the piriformis and obturator internus tendons remain completely undisturbed throughout the procedure.

The lateral approach (also called the Hardinge or transgluteal approach) takes a different route: it splits or partially detaches the anterior fibres of the gluteus medius and gluteus minimus directly from the greater trochanter. These two muscles are the primary hip abductors — the structures that keep the pelvis level during every step — and their surgical disruption is the defining structural consequence of the lateral approach.

The choice of corridor through the hip, in short, is also a choice about which tissues bear the cost of access.

Which muscles are disrupted — and why it matters for gait

"Will I limp after this?" is often the question patients ask first — and the honest answer is that it depends substantially on which structures the surgery disturbs.

Because the lateral approach enters the hip through the anterior gluteus medius and gluteus minimus, healing of these muscles largely governs how fully gait recovers. CT analysis of anterolateral hip approaches found that gluteus minimus cross-sectional area decreased in up to 94% of patients at one year post-operatively — a figure that persists even as patient-reported scores improve. More tellingly, gluteus medius recovery was the only variable that correlated significantly with clinical outcomes at twelve months, meaning that whatever the implant achieves, the result a patient actually experiences is tied to whether these muscles regain their bulk and function.

When they do not, the clinical picture is Trendelenburg gait: on every step, the pelvis tilts towards the unsupported side rather than staying level, producing the characteristic sideways dip. Patients may need a walking stick for many months, and in some cases the insufficiency persists long-term — requiring sustained physiotherapy, continued use of a walking aid, or more complex intervention.

SPAIRE's structural advantage at this point is anatomical rather than technique-dependent. Because the approach interval lies below the abductor compartment, the gluteus medius and gluteus minimus are never divided, retracted under sustained tension, or reattached at closure. Abductor integrity is structurally preserved from the outset.

A secondary benefit concerns sensory continuity. The intact piriformis and obturator internus tendons contain Golgi tendon organs and muscle spindles — mechanoreceptors that transmit continuous positional feedback to the nervous system. Where these structures are divided, the joint loses that proprioceptive input until biological healing restores the pathway; in SPAIRE, the feedback loop is never broken.

Superior gluteal nerve risk: where it comes from and who faces it

The nerve risk in the lateral approach sits alongside, and compounds, the direct muscle damage already described.

The superior gluteal nerve (SGN), arising from the L4, L5, and S1 nerve roots, supplies the gluteus medius, gluteus minimus, and tensor fasciae latae. It exits the pelvis above the piriformis muscle and fans forward through the same anatomical corridor that the lateral approach traverses to reach the hip joint. The branch supplying the gluteus minimus runs closest to the greater trochanter and is therefore the most exposed during lateral access. A 2024 systematic review and meta-analysis of SGN anatomy confirmed this branch as the single highest-risk structure encountered during lateral-approach hip arthroplasty.

The clinical evidence is specific. A quantitative EMG study of 40 patients who underwent total hip arthroplasty via the modified direct lateral approach found that 37.5% — fifteen patients — had acute abductor denervation at six weeks post-operatively. The mechanism in most cases was nerve stretching during retraction, not direct transection: the SGN was not cut, but traction imposed on the nerve during lateral exposure was sufficient to disrupt its conductivity. For the patient, this distinction has little practical relevance — the consequence is abductor weakness, a positive Trendelenburg sign, and dependence on walking aids. Of the fifteen affected patients, only eight showed improvement by twelve weeks; for the remaining seven, denervation had not resolved within the standard postoperative recovery window.

SPAIRE does not enter the abductor compartment and does not approach the SGN's anatomical territory at any stage of the procedure. The nerve is neither stretched, retracted, nor placed at operative risk. This represents a structural absence of SGN risk — not a statistical reduction — because the territory the nerve occupies is never entered by the posterior approach corridor.

Recovery differences: walking aids, dislocation risk, and early function

Recovery in the weeks after hip arthroplasty divides clearly between approaches — and the differences are apparent from day one.

Systematic review evidence comparing hemiarthroplasty approaches consistently finds that patients operated on via the direct lateral route face higher rates of abductor insufficiency, persistent limping, and continued walking-aid dependence than those treated through a posterolateral corridor. Posterolateral approach patients report better quality of life, lower pain scores, and higher overall satisfaction. The historical caveat is that earlier posterolateral techniques — before muscle-sparing refinements such as SPAIRE were developed — carried an elevated dislocation rate, which led many surgeons to remain with the lateral approach despite its abductor cost.

SPAIRE addresses this trade-off through what is described as the 'strap effect'. The intact obturator internus tendon courses around the lesser sciatic notch and passes directly over the posterior femoral head near its centre of rotation, creating a dynamic biological tether against posterior subluxation. The mechanism is anatomically coherent and is consistent with the low dislocation rates reported in SPAIRE case series. Whether this produces a measurable reduction in dislocation risk when compared head-to-head with the lateral approach specifically — rather than with the standard posterior approach — is a question that awaits formal quantification; the HemiSPAIRE RCT and the HIPSTER trial will provide that data, but it is not yet fully available.

What is already established in clinical practice is the postoperative protocol the strap effect makes possible. SPAIRE patients are discharged without hip precautions: no restriction on bending the hip past 90 degrees, no prohibition on crossing the legs, no requirement for raised toilet seats or specialist chairs. Combined with the proprioceptive continuity afforded by the intact short external rotators — described in the preceding section — this allows rehabilitation to proceed from a normal movement baseline rather than from a restricted one.

The contrast for patients is concrete: the lateral approach carries both the recovery burden of abductor disruption and a period of imposed movement rules; SPAIRE's design, mechanistically, removes both.

The current trial evidence and what is still emerging

Three formal trials are now measuring the outcomes that mechanistic evidence alone cannot settle.

The HemiSPAIRE study — an NIHR-funded, two-arm, assessor-blinded RCT — recruited 228 patients between November 2019 and April 2022, comparing the SPAIRE technique directly against the standard lateral approach (then NICE's recommended procedure for hemiarthroplasty following displaced intracapsular fracture). Its primary endpoint is Oxford Hip Score at postoperative day 120, with secondary outcomes covering mobility index, pain, and EQ-5D-5L quality of life. This is the first randomised head-to-head comparison of SPAIRE against the lateral approach.

The HIPSTER trial extends the question to elective total hip arthroplasty. Funded by the NIHR Efficacy and Mechanism Evaluation Programme, it is a three-arm, double-blinded RCT comparing SPAIRE, piriformis-sparing, and a standard robotic posterior approach — with mobilisation speed and return to unrestricted function as its focus. The APOLLO multicentre RCT (555 patients) adds further weight to the posterolateral versus direct lateral comparison in cemented hemiarthroplasty, with EQ-5D-5L as its primary outcome and abductor insufficiency among its secondary endpoints.

What these trials will quantify — and what is not yet in the published record — is the size of any functional advantage: Oxford Hip Scores at defined follow-up points, walking-aid duration in days, and dislocation rates under controlled conditions. The mechanistic case for SPAIRE's advantages in abductor preservation and SGN avoidance is grounded in anatomy and published EMG evidence; what HemiSPAIRE will add is whether those structural benefits translate into a detectable difference on a patient-reported score at four months — the window in which approach-related effects are expected to be most visible.

Patient suitability and the assessment process

The question is not simply which approach performs better in the abstract — it is which approach suits this patient's anatomy, history, and clinical goals.

SPAIRE's muscle-sparing design confers the structural advantages described in earlier sections, but individual factors govern whether it is the right choice. Body habitus, bone quality, the condition of surrounding soft tissue, and specific implant requirements all influence approach selection at the planning stage. In revision settings — where the abductor mechanism may already be compromised, or where gluteus medius reconstruction forms part of the operative plan — the lateral approach remains a clinically appropriate choice. Prior surgery around the posterior hip, or anatomical variants that limit safe posterior access, may similarly favour a different corridor even when SPAIRE would otherwise be suitable. Approach selection is a surgical judgement, not a fixed protocol.

Where SPAIRE is under consideration, pre-operative assessment should cover hip joint anatomy, the functional status of the abductor muscles, and the patient's current gait pattern. Objective gait analysis — such as MAI Motion® biomechanical assessment — can quantify abductor function before surgery, establishing a concrete baseline against which postoperative recovery can be tracked rather than estimated.

Prof Paul Lee leads hip arthroplasty and SPAIRE assessment at Hip Replacement Lincolnshire, which sees patients without referral at centres in Sleaford (NG34) and Grantham (NG31); further information is at hipreplacementlincolnshire.co.uk. For patients weighing up approach-related risks and recovery expectations, a specialist assessment translates the general principles explored in this article into a recommendation grounded in individual anatomy — which is ultimately where the decision has to be made.

1. [1] Hip abductor dysfunction following total hip arthroplasty by modified direct lateral approach: Assessment by quantitative electromyography. (2023). https://doi.org/10.4103/eoj.eoj_99_23 https://doi.org/10.4103/eoj.eoj_99_23
2. [2] Posterolateral or direct lateral approach for hemiarthroplasty after femoral neck fractures: a systematic review. (2020). https://doi.org/10.1177/1120700020931766 https://doi.org/10.1177/1120700020931766
3. [3] Variability and clinical anatomy of the superior gluteal nerve—A systematic review and meta-analysis. (2024). https://doi.org/10.1111/joa.14174 https://doi.org/10.1111/joa.14174
4. [4] THE SPAIRE TECHNIQUE ALLOWS HIP ARTHROPLASTY WITH DIVISION OF ONLY THE OBTURATOR EXTERNUS TENDON. (2018).
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