What the piriformis tendon is

Tucked beneath the gluteal muscles and invisible from outside the body, the piriformis is a flat, pyramidal muscle that runs from the front of the sacrum — the triangular bone at the base of the spine — through the greater sciatic foramen and attaches via its tendon to the greater trochanter, the bony prominence at the outer top of the thigh.

It is one of six short muscles known collectively as the lateral rotator group, sometimes called the 'quadriceps coxa' of the hip. Together, these muscles form the posterior musculotendinous sleeve of the joint, governing external rotation and contributing to the structural integrity of the hip's back wall.

Sitting immediately alongside the piriformis is its anatomical neighbour, the obturator internus. This muscle follows an unusual path: it exits the pelvis through the lesser sciatic foramen, bends around the lesser sciatic notch in a pulley-like arc, and inserts on the medial surface of the greater trochanter — passing directly over the posterior femoral head, close to the joint's true centre of rotation.

That anatomical trajectory, unremarkable in a healthy hip, becomes surgically significant the moment a posterior incision is planned — which the following sections explain.

What the piriformis tendon does in a healthy hip

The piriformis has a straightforward primary job: it rotates the femur externally, turning the leg and foot outward from the hip. Working alongside the other five short external rotators — obturator internus, obturator externus, gemellus superior, gemellus inferior, and quadratus femoris — it braces the back of the hip joint and actively resists unwanted inward rotation of the thigh.

That resistance matters most in a very specific movement combination. When the hip flexes and the leg simultaneously rotates inward — sitting down heavily, crossing the legs, or leaning forward with the feet turned in — the posterior joint capsule and femoral head are placed under strain. The piriformis and the obturator internus, positioned together over the back of the joint as a paired muscular sling, generate tension that counteracts this strain. Even at rest, their passive viscoelastic pull helps keep the femoral head properly seated in the socket.

This is precisely the movement pattern most associated with dislocation after posterior hip surgery: combined flexion and internal rotation. The piriformis and obturator internus are the structures best placed, anatomically, to resist it — which is why what happens to their tendons during a surgical approach carries real consequences for early stability.

What conventional posterior hip replacement does to it

Gaining access to the hip joint through a posterior incision requires the surgeon to work through the very structures described in the previous section. In the conventional posterior approach, the piriformis tendon — along with part or all of the short external rotator tendons — is released from its insertion on the greater trochanter to allow the instruments and implants to reach the joint. The release is deliberate, controlled, and standard practice; it does not represent a surgical error. It does, however, involve a trade-off.

Once the tendon is divided, two things change. First, the mechanical contribution of the piriformis — the passive tension and active contraction that help seat the femoral head and resist combined flexion and internal rotation — is lost until biological healing restores it. The duration of that healing is not fixed; it varies between patients and cannot be confirmed with certainty from the outside. Second, the Golgi tendon organs and muscle spindles embedded within the tendon are interrupted, removing the afferent proprioceptive signals they normally relay. Without those signals, the hip cannot instinctively guard itself; the patient must rely on conscious awareness instead.

Postoperative hip precautions — typically advice to avoid bending the hip beyond 90 degrees, crossing the legs, or rotating the foot inward — exist largely because of this window. They compensate, through behavioural rules, for the mechanical and sensory protection that the intact tendon would otherwise provide.

The strap effect: how intact tendons resist dislocation

The SPAIRE technique's answer to those consequences lies in a principle that becomes the operative logic of the whole approach: if the tendons that naturally protect the posterior hip are never cut, their protective effect never lapses.

When the piriformis and obturator internus are left intact, the combined structure acts as what the clinical literature describes as a 'strap effect' — a continuous posterior tether across the back of the replaced joint. Because of the obturator internus's anatomical trajectory directly over the posterior femoral head (described in the anatomy section above), its preserved tension runs at almost exactly the right angle to resist the femoral head translating backwards out of the socket. The piriformis reinforces this from its own insertion on the greater trochanter.

Two forms of tension contribute simultaneously. Passive viscoelastic pull is present even at rest — a constant baseline force keeping the prosthetic head seated in the socket. Active muscular contraction then adds to this during movement, generating resistance proportional to the load placed on the joint. Both are directed against the same dislocation vectors already described, making the strap effect biomechanically well-matched to the risk.

The practical consequence concerns timing. No healing gap needs to close because no tendon was divided — meaning this resistance may be present from the first postoperative day in selected patients following the SPAIRE pathway, rather than depending on biological repair of cut tissue.

It is worth being precise about what this means and does not mean. The strap effect is a biomechanical argument grounded in anatomical description; it does not eliminate dislocation risk. Clinical comparative data on piriformis-sparing approaches continues to accumulate, and outcomes depend on individual anatomy, implant choice, and recovery factors.

Proprioceptive continuity: the sensory case for preservation

Mechanical resistance, described in the previous section, is a force-based argument. The sensory case for piriformis preservation is different in kind — it concerns not how the joint is held in place, but how it knows where it is.

When the piriformis and obturator internus tendons stay intact, the mechanoreceptors embedded within them continue transmitting uninterrupted. The joint retains what might be described as automatic self-awareness: moment-to-moment positional data arriving at the brain before any conscious decision is made. If the hip moves toward a risky range, the corrective muscular response may begin reflexively — faster than attention can intervene.

In the weeks after surgery, when concentration flags, routine tasks become automatic, and patients stop thinking about every step, that reflex layer may carry genuine practical weight. Conscious precautions depend on memory and attention; reflex protection does not.

No precise timeframe exists for sensory recovery after tendon division. Neural regeneration following soft-tissue surgery varies considerably between individuals and cannot be confirmed from the outside. SPAIRE avoids the question entirely: because no tendon is cut, no recovery period is needed before this sensory system can resume its role.

Roger and Hill (2012) provided independent support for the functional value of this approach, observing that retaining the short external rotators during posterior hip replacement may confer clinical benefits in patient recovery — a finding framed around surgical outcomes rather than biomechanical modelling alone, and one of the earlier academic acknowledgements that what is preserved at the time of surgery may be as consequential as what is implanted.

How Professor Paul Lee applies piriformis preservation in practice

The arguments assembled across the preceding sections — anatomical, mechanical, and sensory — resolve into a single operative decision: whether to cut the posterior tendons or preserve them. The technique that makes preservation its defining commitment is SPAIRE, first described by Kim et al. in 2008 as the Modified Posterior Approach.

The acronym carries two overlapping definitions. For patients, it stands for 'Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus.' In the academic literature it appears as 'Soft-tissue Preserving, Anatomic, Internus muscle Retaining, External rotator preserving.' Both framings centre on the same intraoperative commitment: the piriformis and obturator internus tendons are left entirely undisturbed throughout the operation.

Professor Paul Lee, who trained at the Exeter Hip Unit under Professor Timperley and has published on the technique's advantages, has made SPAIRE his routine approach for total hip replacement. One practical consequence that extends beyond stability is surgical precision: with the native posterior soft-tissue tension intact, the surgeon retains direct tactile feedback during the operation, which may assist judgement of leg length and femoral offset in real time — a benefit that sits alongside, rather than separate from, the strap effect and proprioceptive arguments already described.

Suitability depends on individual anatomy, implant selection, and clinical factors that vary between patients — and any decision about approach should follow a proper specialist assessment. What the anatomy in this article makes clear, however, is the underlying logic: a piriformis tendon that was never cut does not need to heal before it can work, and that difference may matter most in the first days after surgery, when the joint is finding its feet.

1. [1] Piriformis muscle — Wikipedia. https://en.wikipedia.org/?curid=1104396 https://en.wikipedia.org/?curid=1104396
2. [2] List of external rotators of the human body — Wikipedia. https://en.wikipedia.org/?curid=8986462 https://en.wikipedia.org/?curid=8986462