What the posterior approach to hip replacement involves

Total hip replacement involves removing the worn surfaces of the ball-and-socket joint and fitting a prosthetic cup into the pelvis and a stem into the upper femur. The implants themselves — the cup, stem, and bearing surface — are well-established components used across all surgical approaches. What varies between techniques is not the prosthesis but the path the surgeon takes to reach the joint, and how the surrounding muscles and tendons are managed along the way.

The posterior approach is the most widely used route in hip replacement worldwide. The surgeon approaches the hip from behind, which gives a clear view of the joint and generous room to work. To open up this access, several small tendons at the back of the hip — including the piriformis and the obturator internus, collectively known as the short external rotators — are detached from the bone. Once the new implants are in place, these tendons are reattached before the wound is closed.

Reattaching severed tissue is routine in surgery, but biological healing takes time. After a standard posterior hip replacement, the repaired tendons require up to 90 days to regain their natural strength and integrity. During that window, the back of the hip is temporarily less stable than it will eventually become — a factor that has shaped how post-operative care has traditionally been designed.

What SPAIRE actually changes

SPAIRE is an acronym that describes precisely what the technique does: Save Piriformis And (obturator) Internus, Repair (obturator) Externus. Unpacking those four words reveals the entire operative difference from a standard posterior approach.

In conventional posterior surgery, all of the short external rotators — including the piriformis and the obturator internus — are detached to expose the joint, then reattached at closure. SPAIRE alters only the soft-tissue handling:

• Piriformis tendon — left completely intact throughout the operation; it is neither cut nor released at any point.
• Obturator internus tendon — also preserved in full; like the piriformis, it remains attached to the bone from start to finish.
• Obturator externus — deliberately detached to create a controlled 'trap door' into the joint, then meticulously repaired before closure.

Because the piriformis and obturator internus are never divided, they carry no healing burden after surgery. Their continuity with the bone is unbroken.

The technique was originally described by Kim et al. in 2008 as the Modified Posterior Approach. Professor Timperley at the Exeter Hip Unit subsequently formalised and published it in peer-reviewed literature in 2017 (PubMed 28218374). The prosthesis — cup, stem, and bearing surface — is the same as in any other posterior hip replacement; the change is entirely in how the surgeon navigates the surrounding soft tissue.

Why keeping the tendons intact may improve stability

Two interconnected biological mechanisms help explain why leaving the piriformis and obturator internus intact may make the reconstructed joint more stable from the moment the wound is closed.

The first is what surgeons describe as the strap effect. The obturator internus tendon takes a curved path around the back of the pelvis before anchoring into the femur — a geometry that, when the tendon is intact, acts much like a living seatbelt across the rear of the joint. Any tendency for the new femoral head to slide backwards is met with immediate passive resistance from that unbroken tether. In a standard posterior replacement, this resistance is absent for weeks while the reattached tendon heals.

The second mechanism operates at a neurological level. Embedded within healthy tendons are Golgi tendon organs and muscle spindles — tiny sensory receptors that continuously report joint position to the brain. When those tendons are divided and reattached, this feedback is interrupted for as long as tissue regeneration takes. SPAIRE keeps the receptors in circuit throughout: the hip retains something like an internal GPS that may prompt the surrounding muscles to guard against extreme positions instinctively, rather than relying on conscious compliance with movement rules.

Together, these two mechanisms are thought to underlie the low dislocation rates reported in SPAIRE case series — though it should be said that most of the supporting evidence is observational rather than from head-to-head randomised trials, so the biomechanical reasoning remains better established than the long-term comparative numbers.

There is a practical surgical benefit as well. Intact posterior tension gives the operating surgeon more consistent tactile feedback when judging leg length and femoral offset during the procedure, which may help reduce the small but meaningful risk of a postoperative limb-length discrepancy.

Hip precautions — why standard surgery needs them and SPAIRE may not

For many patients, the words 'hip precautions' land harder than the prospect of surgery itself. After a standard posterior replacement, the operated hip must not flex beyond 90 degrees — roughly the angle of a standard dining chair — for up to 12 weeks. In practice, that rule reshapes dozens of ordinary movements: getting in and out of a car requires planning, low sofas become hazards, a bath is off limits, and putting on shoes or socks may need an adapted technique or a carer's help.

The restriction exists for a clear biological reason: the posterior tendons that were divided to expose the joint must reattach and regain strength before they can safely restrain the new prosthesis. During that vulnerable healing window, one careless movement beyond the permitted range can lever the femoral head out of the cup. The consequences are rarely straightforward: 57% of patients who experience a first dislocation go on to dislocate again, and 45.6% require revision surgery within two years.

SPAIRE addresses this at the root rather than managing around it. Because the piriformis and obturator internus are never divided, there is no repair-dependent window to bridge — the posterior structures are mechanically continuous from wound closure onwards. In suitable patients, this removes the biological justification for strict precautions rather than simply compensating for their absence.

Within a planned pathway, that intact stability may support earlier confident mobilisation and, for appropriately selected patients, a same-day or next-day discharge. Individual recovery still depends on anatomy, rehabilitation engagement, and the surgeon's intraoperative findings. Professor Paul Lee — a Consultant Orthopaedic Surgeon whose SPAIRE practice and rapid recovery programme are described in the sections that follow — has structured his approach to hip replacement around precisely this principle.

How SPAIRE sits alongside other surgical approaches

Several legitimate surgical routes exist for total hip replacement, each with genuine advantages and real trade-offs. No single approach suits every patient or every surgeon.

The direct anterior approach (DAA) works through a muscle interval at the front of the hip, largely avoiding the posterior structures altogether. That corridor can support fast early recovery, but the technique carries a risk of lateral femoral cutaneous nerve injury — producing lasting numbness down the outer thigh — alongside longer operative times, higher intraoperative blood loss, and a learning curve that makes outcomes more variable when a surgeon is building experience with it.

The lateral (Hardinge) approach enters from the side and sidesteps the posterior anatomy, but requires partial release of the gluteus medius — the main abductor muscle — which can affect hip abductor strength during recovery.

The standard posterior approach offers reliable, broad exposure and is technically familiar to most hip surgeons, but historically carries a higher dislocation rate and, as earlier sections explain, requires movement precautions while the reattached tendons heal.

SPAIRE occupies a specific position in this landscape: it uses the same posterior corridor, retaining its familiarity and extensibility, while addressing the main historical weakness of that route by keeping the key stabilising tendons intact. A 2025 study by Paus et al. found that SPAIRE patients achieved better short-term functional scores — including walking speed and the Short Physical Performance Battery — than those operated via the lateral approach, though this is one comparative data point rather than a definitive verdict. Surgeons select an approach based on their training, the individual patient's anatomy, and what the intraoperative picture demands.

What to expect if SPAIRE is recommended for you

Reaching this stage of the decision usually means imaging has confirmed significant joint damage and conservative options have been exhausted. The next step is individual assessment — SPAIRE suitability depends on anatomy, body habitus, and what the surgical planning shows, and is not assumed for every candidate.

Professor Paul Lee trained directly under Professor Timperley at the Exeter Hip Unit, where the SPAIRE technique was formalised and published in peer-reviewed literature, and has made it his routine approach to total hip replacement. Around it he has built a structured pathway that integrates surgical planning, implant selection, and mobilisation design — one that aims to support same-day or next-day discharge for appropriately selected private patients, though the timeline always depends on individual recovery.

Risks remain. Infection, blood clots, and dislocation are possible after any hip replacement, and no technique removes them. What SPAIRE may change is their relative profile — particularly the dislocation risk during the first weeks — and the conditions under which early recovery begins.

For patients who are suitable, the difference that intact posterior tendons make in practice tends to show earliest not in a clinical score but in the small ordinary things: being able to sit down in the recovery room without calculating the angle of the chair, moving to the bathroom on the first day without mapping every step against a list of forbidden positions. That freedom is the direct consequence of a hip that retains its own structural tether from the moment surgery ends.