Two routes to the same joint

If your surgeon has mentioned both SPAIRE and the anterior approach, the most practical question is simply: which side of the hip does each one come from, and why does that matter?

SPAIRE — which stands for Saving Piriformis And (Obturator) Internus with Repair of (Obturator) Externus — enters the hip from behind, through a posterolateral corridor. Its defining feature is what it does not cut: the piriformis and obturator internus tendons remain fully intact throughout the operation. Only the obturator externus tendon is divided and then repaired. Those preserved tendons do more than survive the procedure — they serve as live anatomical landmarks, giving the surgeon direct tactile feedback on leg length and joint tension as the new implant is seated. First described in 2016 at the Exeter Hip Unit, SPAIRE is now routinely combined with MAKO robotic-arm assistance, and it is the approach used by the specialist team at Hip Replacement Lincolnshire.

The Direct Anterior Approach (DAA) works from the opposite direction. The surgeon enters through an intermuscular plane at the front of the hip, slipping between the tensor fasciae latae and the sartorius without detaching either. Because it is a genuinely anterior route, the posterior tendons are never encountered — but the approach passes close to the lateral femoral cutaneous nerve and, more rarely, the femoral nerve.

Both techniques are classed as muscle-sparing alternatives to the standard posterior approach, which typically cuts all of the short external rotators. The difference between SPAIRE and DAA is not one of ambition — both aim to reach the joint with less soft-tissue disruption — but of corridor. One goes round the back; the other comes in from the front. That single anatomical choice shapes nearly every tradeoff discussed in the rest of this article: nerve risk, intraoperative references, equipment needs, and patient suitability. The implant itself is unchanged by either route.

Nerve risk: what each approach puts at stake

Any hip replacement carries a small but real risk of nerve injury — the question is which nerve, and where it sits in relation to the surgical corridor.

With the Direct Anterior Approach, the nerve most commonly affected is one that runs across the front and outer aspect of the thigh, supplying sensation to the skin there. Surgeons and anatomists call it the lateral femoral cutaneous nerve (LFCN). Because it crosses the operative field when the hip is approached from the front, it can be stretched or bruised even during careful surgery. A prospective trial of 195 DAA cases found LFCN injury in 21.9% of patients with a standard longitudinal incision and in 36.4% with a bikini-style incision variant; most cases resolved within six months, but 11 patients still had persistent sensory symptoms at final follow-up. The result is typically numbness or tingling on the outer thigh — uncomfortable, and occasionally permanent, though rarely disabling. The main femoral nerve, which controls the quadriceps and sensation over the inner thigh, sits deeper and is injured far less often with DAA, but when it is affected the consequences are more serious.

SPAIRE and other posterolateral approaches sidestep the anterior neurovascular territory entirely. Their background nerve risk involves the sciatic nerve, which runs behind the hip joint. Across all approaches, nerve injury occurs in roughly 0.6–3.7% of primary hip replacements, rising to around 7.6% in revision cases or where there is developmental dysplasia. Because SPAIRE preserves the posterior capsular and tendinous structures rather than stripping them back, it may reduce the degree of posterior soft-tissue retraction and the traction placed on the sciatic nerve — but this does not eliminate the risk, and sciatic nerve injury remains the most common nerve complication in hip arthroplasty as a whole.

Neither route is nerve-risk-free. The profiles differ in location and character: a front-of-thigh sensory issue with DAA versus a posterior motor-and-sensory risk with posterolateral techniques. For patients with specific concerns about either, an individualised assessment is the appropriate next step.

Recovery speed and functional outcomes

The claim that anterior hip replacement means faster recovery has a genuine basis in published data — but the advantage is largely confined to the first few weeks.

In the immediate postoperative period, DAA consistently demonstrates earlier mobilisation, shorter hospital stays, and better scores on measures such as the Harris Hip Score in the six to twelve weeks following surgery. For patients in appropriate anatomy, operated on by a surgeon with sufficient experience in the technique, that early edge is real and should not be dismissed.

By three to twelve months, the gap closes. Functional outcomes converge across modern approaches, and no sustained long-term advantage has been demonstrated for either DAA or SPAIRE beyond that early window. Implant survival and all-cause revision rates are similarly equivalent — neither technique confers a longevity benefit over the other.

On the SPAIRE side, a 2025 study found that patients who had their hip replacement via the SPAIRE approach achieved better scores for neuromuscular status (NMS), Short Physical Performance Battery (SPPB), and walking speed than those treated via the lateral approach. That finding is worth noting for its functional signal, but it is a comparison with the lateral technique rather than a direct head-to-head study against DAA, and it should be read in that context.

Definitive head-to-head RCT evidence comparing SPAIRE with DAA specifically does not yet exist. The NIHR-funded HIPSTER trial is currently recruiting and will be the most rigorous source of comparative data when published — until then, the available evidence base rests on systematic reviews, registry data, and observational studies.

Dislocation risk and movement restrictions

'Will I be able to sit in a low chair?' and 'Can I cross my legs?' are among the most common questions patients ask before hip replacement — and the answer depends partly on which approach is used, because the directions of risk differ between SPAIRE and DAA.

With SPAIRE, the piriformis and obturator internus tendons remain intact and under normal physiological tension at the back of the joint. They act, in effect, like a natural seatbelt across the posterior capsule — resisting the femoral head from slipping backwards. Golgi tendon organs and muscle spindles within these preserved structures continue to send afferent proprioceptive signals to the nervous system, providing moment-to-moment feedback that may reflexively guard against extreme positions. The practical result is that posterior dislocation risk is reduced, and patients typically face fewer — and less stringent — restrictions on movements such as sitting back in a deep chair or bending gently at the hip.

DAA patients do not face the same posterior vulnerability, but they are generally advised to avoid extreme hip flexion and certain anterior loading positions in the early weeks, because the anterior soft-tissue envelope has been disturbed rather than the posterior one. The direction of restriction is reversed, not eliminated.

Both approaches represent an improvement on the standard posterior technique, where all the short external rotators are divided and the dislocation precaution regime is correspondingly more demanding. With either SPAIRE or DAA, the early restriction period is shorter and less restrictive than patients who had a standard posterior hip replacement a decade ago might remember or have been told about.

For patients whose lifestyle involves habitual extreme hip flexion — certain occupational postures, for example — a modified SPAIRE technique has been described to address elevated anterior dislocation risk, demonstrating that the approach can be adapted rather than simply ruled out. The appropriate technique for any individual depends on a full clinical assessment of anatomy, activity level, and personal risk profile.

Which patients suit each approach

Choosing between SPAIRE and DAA is not a matter of one being better than the other — it is a matter of which fits the individual patient standing in front of the surgeon.

DAA is more constrained by body shape and anatomy. Patients with a higher BMI, shorter femoral necks, or a narrower hip canal present greater operative difficulty on the anterior approach, and the risk of a hairline fracture during canal preparation rises on the learning curve. Certain hip geometries — reduced offset in particular — make positioning and femoral preparation through the anterior corridor technically demanding even in experienced hands.

SPAIRE accommodates a wider range of body types and pelvic morphologies. That said, it is not without its own challenging subgroups: patients with small anatomy, reduced femoral offset, or a fixed external rotation deformity require more precise surgical planning. In these cases, MAKO robotic assistance is routinely used alongside the SPAIRE technique to achieve accurate implant positioning where the soft-tissue exposure alone would limit the surgeon's visual reference.

Previous surgery around the hip, the presence of significant deformity, and the patient's rehabilitation goals all further shape the decision. Equipment availability is also a practical factor: DAA depends on a specialised traction table; SPAIRE does not, which can matter in different hospital or day-surgery settings.

Activity goals and habitual posture — discussed in the previous section — feed into this conversation too. A specialist assessment that includes imaging and, where relevant, biomechanical evaluation is how these variables are weighed individually, rather than through any simple checklist.

How the decision gets made in practice

Three factors tend to be decisive when surgeon and patient work through approach selection together: the geometry of the hip on imaging — offset, neck angle, and canal shape; the patient's habitual movement patterns and their implications for instability risk direction; and the surgeon's accumulated experience with each technique, since both SPAIRE and DAA require genuine technical investment to deliver their claimed benefits consistently.

It is worth being precise about the current evidence position. The HIPSTER trial, noted earlier for its design, compares robotic-assisted SPAIRE and piriformis-sparing approaches against a standard posterior control — not against DAA. Even when its results are published, a direct randomised comparison of SPAIRE versus DAA will remain to be done. Choosing between these two approaches currently means working from systematic reviews, registry data, and observational cohorts rather than head-to-head trial data.

Hip Replacement Lincolnshire accepts patients without a GP referral. Consultant-led assessments are available at Sleaford (NG34) and Grantham (NG31) — hipreplacementlincolnshire.co.uk.

The most useful preparation for a pre-operative consultation is a short inventory: the activities the patient wants to return to, any positions the hip is routinely placed in, and a clear account of any previous surgery around the joint. Prof Paul Lee's assessment framework uses imaging review and this kind of functional mapping to match the surgical corridor to the individual — the goal being to identify the right approach for a specific anatomy and set of goals, rather than to apply a single technique uniformly.

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