Why the surgical approach matters for muscle disturbance

Most patients focus on the implant — the ceramic ball, the titanium socket, the bearing surface — when trying to understand what makes one hip replacement different from another. In practice, it is the surgical approach, not the prosthesis, that determines which muscles and tendons are touched, repaired, or left completely undisturbed. The implant sits inside the joint; the approach is the route taken to reach it.

The hip joint itself is a deep ball-and-socket articulation — the rounded femoral head sitting within the acetabulum of the pelvis — and reaching it safely means navigating layers of muscle, tendon, and capsule. Four approaches are in common clinical use: the posterior approach (the most widely performed in the United States and United Kingdom), the direct anterior approach (DAA), the direct lateral (Hardinge) approach, and SPAIRE — a muscle-sparing modification of the posterior route whose name stands for Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus.

The critical soft-tissue decision in each approach comes down to two groups of structures: the short external rotator tendons (principally the piriformis and obturator internus, which stabilise the hip and feed proprioceptive signals to the nervous system) and the hip abductor tendons (gluteus medius and minimus, which control balance and walking gait). Whether these are cut, split, detached, or preserved whole shapes recovery pace, the need for post-operative hip precautions, dislocation direction and risk, and longer-term functional outcomes — independent of which implant is chosen.

Across the four approaches there is a clear spectrum, from the lateral technique — which disturbs the abductor mechanism most — through the standard posterior, to the DAA and SPAIRE, which each aim to minimise soft-tissue disruption by different anatomical strategies.

Posterior approach: rotators released, abductors preserved

The real tissue decision in a standard posterior approach concerns the short external rotators — a group of small but functionally important tendons that wrap around the back of the hip joint and attach to the greater trochanter of the femur. The principal structures in this group are the piriformis, the gemelli (superior and inferior), the obturator internus, and the obturator externus. Together they rotate the thigh outward, contribute to joint stability, and — critically — carry proprioceptive nerve endings that relay position and movement information to the central nervous system. To reach the posterior hip capsule and safely dislocate the joint for implant insertion, all four tendons must be detached from the greater trochanter.

The gluteus medius and minimus — the primary abductor muscles that keep the pelvis level during walking — are entirely untouched by the posterior approach. This is a meaningful distinction: because the abductor mechanism is left intact, patients do not experience the abductor weakness and associated limp that can follow the lateral (Hardinge) approach. Preserving these muscles is one of the reasons the posterior route remains the most commonly performed approach in both the United States and the United Kingdom.

Historically, releasing the short external rotators carried an elevated risk of posterior dislocation, because those tissues normally provide a passive restraint behind the joint. Two developments have substantially narrowed this risk. First, careful repair of the detached rotator tendons and posterior capsule back to the greater trochanter at the end of the procedure restores the posterior soft-tissue envelope. Second, the routine use of larger femoral head components (32–36 mm) increases the arc of movement needed to lever the ball out of the socket. Together, these changes have brought posterior dislocation rates much closer to those of other approaches.

Hip precautions remain standard while repaired tissues consolidate: patients are typically advised to avoid bending the hip past 90°, crossing the legs, or rotating the foot inward for several weeks post-operatively. These restrictions reflect the biology of tendon healing — the tissues have been released and reattached, not preserved in continuity.

Direct anterior approach: working between muscles, not through them

Between the sartorius — a long strap muscle running down the front of the thigh — and the tensor fasciae latae at the outer hip, the body already provides a natural corridor into the joint. The direct anterior approach (DAA) exploits this gap precisely: the surgeon works between these two muscles rather than through any of them, so no tendon is released and no muscle belly is cut across. Surgeons sometimes describe it as finding a door that was already there.

Because the approach never reaches the back of the hip, the posterior capsule and the entire group of short external rotators — the same tendons detached in a standard posterior procedure — remain completely undisturbed. There is therefore no soft-tissue reason for the hip to dislocate backwards, and most DAA patients are not given the movement restrictions that apply while repaired posterior tissues consolidate. The absence of post-operative hip precautions is a practical benefit for day-to-day activity in the weeks following surgery.

This profile comes with two genuine trade-offs worth understanding. First, if dislocation does occur, it tends to happen anteriorly — forward rather than backward — because the front of the joint has been entered and the anterior capsule disturbed during implant insertion. Second, the gluteus medius and minimus — the abductor tendons that control pelvic balance during walking — cannot be visualised or surgically addressed through an anterior window. In patients who also have abductor-tendon pathology, this is a meaningful clinical constraint: the approach that spares the posterior structures by design cannot access the superior ones. Individual anatomy and any co-existing hip pathology therefore influence whether the DAA is the most appropriate route, and a specialist assessment is needed to weigh those factors.

Lateral approach: the abductor trade-off

The lateral (Hardinge or transgluteal) approach sits at the opposite end of the soft-tissue spectrum from the direct anterior route. Rather than preserving the posterior joint and leaving the superior muscles alone, it does the reverse: the short external rotators and posterior capsule are left completely intact, but the abductor muscles — the structures most responsible for a normal walking pattern — are deliberately divided to reach the joint.

What is divided and what is spared

Accessing the hip from the side requires splitting the anterior third of the gluteus medius longitudinally and either splitting or detaching the gluteus minimus beneath it. The vastus lateralis is elevated distally. One firm anatomical boundary governs the proximal extent of the medius split: it must not extend more than 5 cm above the tip of the greater trochanter. The superior gluteal nerve enters the muscle belly just beyond that point, and damage here can produce lasting abductor weakness that rehabilitation alone is unlikely to fully resolve.

The dislocation advantage

Because the piriformis, obturator internus, and posterior capsule are never disturbed, the lateral approach carries one of the lowest dislocation rates of any standard technique. There is no posterior soft-tissue deficit in the early post-operative weeks, and hip movement precautions are not generally required.

The gait cost

The trade-off is meaningful in daily life. Research suggests that patients undergoing the lateral approach report limping approximately twice as often at one to three years post-surgery compared with those who have had anterior or posterolateral procedures. The mechanism is Trendelenburg gait: a weakened gluteus medius cannot hold the pelvis level during the stance phase of walking, so the opposite side dips with each step — a limp that is visible in everyday movement and may persist for months or longer in a proportion of patients.

The approach retains genuine value in certain anatomical settings, and patient-specific factors always inform which route is most appropriate — a decision that requires individual consultant assessment.

SPAIRE: preserving the rotators the standard posterior approach cuts

SPAIRE — an acronym for Saves Piriformis And Obturator Internus, Repairs Obturator Externus — begins with the same posterior skin incision and tissue corridor used in a standard posterior hip replacement. The operative route is not new. What changes is the decision made once the posterior capsule comes into view: rather than detaching the piriformis and obturator internus from the greater trochanter to gain access, the surgeon works around them, keeping both tendons intact throughout the procedure.

The strap effect

The anatomical consequence of this choice is immediately mechanical. The obturator internus exits the pelvis via the lesser sciatic foramen and courses directly over the posterior aspect of the femoral head. When preserved, it acts as a biological tether — its active contraction and passive viscoelastic tension both resist posterior displacement of the femoral head within the socket. Surgeons describe this as the 'strap effect': the intact tendon provides a natural restraint that supplements the mechanical stability of the prosthetic components without depending on the quality of a repair to a detached structure.

Position sense without a healing interval

Intact tendons retain their embedded Golgi tendon organs and muscle spindles — the sensory receptors that continuously signal hip position and load to the central nervous system. After a standard posterior hip replacement, these mechanoreceptors are severed when the tendons are detached; the hip operates with reduced proprioceptive input until biological healing and neural regeneration occur. By preserving the short external rotators, SPAIRE maintains this positional feedback from day one, meaning the central nervous system receives uninterrupted kinesthetic information through the early recovery period.

The clinical profile associated with the technique includes no requirement for post-operative hip precautions, very low dislocation rates, and faster biological recovery than standard posterior hip replacement — outcomes consistent with the reduced degree of soft-tissue disruption. Preservation of the native tendinous attachments also provides reliable intraoperative reference for judging leg length and femoral offset, supporting accurate component positioning.

Prof Paul Lee performs SPAIRE as a modified posterior technique within a consultant-led assessment pathway. Like any surgical approach, it is not appropriate for every patient, and individual anatomy, prior surgery history, and functional goals all inform whether SPAIRE, a standard posterior, the direct anterior approach, or another route is the most suitable choice.

Which approach suits which patient — and what to ask

Choosing between approaches is ultimately a matter of risk distribution — which tissues can best tolerate disturbance given a particular patient's anatomy, surgical history, and functional expectations.

Running from least to most soft-tissue disruption: SPAIRE and the direct anterior approach occupy the same end of the spectrum, though via different anatomical corridors. Standard posterior sits in the middle — short external rotators released and repaired, abductors fully intact. The lateral approach disturbs the most soft tissue, particularly the abductors, while leaving the posterior structures undisturbed.

The central clinical trade-off is dislocation risk against muscle and tendon trauma. SPAIRE is the only approach that addresses both at once, preserving the very structures that resist posterior displacement rather than cutting and then relying on their repair. The direct anterior approach achieves low posterior dislocation risk through a different mechanism — an intact posterior capsule — but with different anatomical constraints.

What influences approach selection

Several patient-specific factors bear on this decision:

• Body habitus and hip extension range can limit access via the direct anterior approach, which requires specific intraoperative leg positioning.
• Prior lumbar spinal fusion alters pelvic tilt and materially increases dislocation risk across all approaches, favouring techniques that maximise soft-tissue stability.
• Existing abductor pathology may make a lateral approach unsuitable for patients whose gluteus medius is already compromised.
• Surgeon training and available instrumentation are themselves outcome determinants — no approach should be performed without the relevant experience.

Long-term comparative outcome data across all approaches, at scale, remain limited. That is a genuine feature of the current evidence, present throughout the discussion above, not a footnote appended at the end.

Questions worth raising before surgery

Patients can usefully ask: which approach do you use, and why for my anatomy specifically? Which tendons or muscles will be disturbed? Will I need hip precautions, and for how long? What is your dislocation rate with this technique?

A consultant who performs more than one approach and can weigh individual anatomy against the evidence is best placed to answer those questions honestly. Hip Replacement Lincolnshire, part of the MSK Doctors group, offers that assessment without a referral.