Why your surgical approach decides your movement rules

What you can and cannot do after hip replacement depends less on the operation itself and more on exactly how the surgeon reached your hip joint.

The hip is a ball-and-socket joint, where the rounded head of the femur sits inside the cup-shaped acetabulum of the pelvis. Holding the ball in the socket is a job shared by the joint capsule — a tough sleeve of fibrous tissue that encloses the joint — and a set of short muscles and tendons known as the external rotators, which include the piriformis and obturator internus. These structures do not simply move the hip; they act as passive stabilisers, keeping the prosthetic ball centred in the socket throughout daily movements.

When a surgeon approaches the hip joint, at least some of this surrounding tissue must be divided to gain access. Which structures are cut determines where the newly replaced joint is temporarily vulnerable. A posterior or posterolateral approach opens the back of the capsule and divides the external rotator tendons, leaving the rear of the joint at risk: the prosthetic ball can lever out backwards if the hip is simultaneously bent, turned inwards, and brought across the body's midline. An anterior approach, by contrast, opens the front of the joint; the vulnerability is reversed, so the risky movements are extension behind the body and outward rotation of the leg.

Because dislocation direction is approach-specific, the movement restrictions that follow are equally specific — there is no single universal rule that applies to every patient.

The rationale for time-limiting these restrictions is biological. Severed capsular and tendinous tissues require roughly 90 days to reattach, fibrose, and regain meaningful tensile strength. Up to 60–80% of all post-operative dislocations occur within this first three-month window, which is why precautions are most strictly enforced early in recovery. Across primary total hip arthroplasty, overall dislocation rates in the published literature range from 0.12% to 16.13% depending on approach, technique, and patient factors, with a commonly cited average of 1–3% for primary procedures.

Understanding why these rules exist — and specifically which approach generated them — is what makes it possible to follow them confidently, and to ask the right questions when assessing the options for surgery.

Posterior approach: the 90-degree triad and what it forbids

Dividing the piriformis, obturator internus and externus, and the posterior capsule creates a specific mechanical hazard at the rear of the socket. With those tendinous stabilisers severed, the prosthetic ball can be levered out of the back of the joint by three movements acting in combination: hip flexion beyond 90°, adduction of the operated leg past the body's midline, and internal rotation of the foot and knee toward the opposite side. Surgeons and physiotherapists refer to this as the 'forbidden triad', and its danger lies in the geometry — each movement alone may be survivable, but together they direct the neck of the prosthesis against the posterior rim of the socket with enough force to displace the ball rearward.

In everyday terms, the triad maps onto a surprisingly routine range of activities. Tying shoelaces is the textbook example of forbidden flexion — leaning the trunk forward past a right angle to reach the foot. Crossing the operated leg over the opposite knee on a sofa violates adduction. Twisting to retrieve something from the back seat of a car, or pivoting on the operated foot without first lifting it, generates internal rotation. Getting out of a low car seat is particularly hazardous because it can combine all three vectors simultaneously and is one of the most commonly reported mechanisms of early post-operative dislocation.

The clinical consequences of a single dislocation are severe enough to justify the precautions. Published data indicate that 57% of patients who dislocate will do so again, 11% will experience more than five separate events, and 45.6% will require revision surgery within two years. Ohio State University's 2022 clinical practice guideline formalises approach-specific restrictions in a standardised protocol, and a 2023 Journal of Arthroplasty study (Christensen et al., cited 46 times) confirmed that dislocation characteristics differ significantly between surgical approaches — supporting the case for approach-specific rules rather than a single generic list applied to all patients.

Anterior and anterolateral approaches: when the risk reverses

The mirror image of the posterior triad sits on the opposite side of the joint. Where a posterior approach divides the back of the capsule, the direct anterior approach (DAA) and anterolateral variants open the front — and it is the anterior capsule that becomes the vulnerable structure as it heals.

The consequence is a complete reversal of the forbidden movements. Rather than guarding against flexion, adduction, and internal rotation, anterior-approach patients must avoid extending the operated leg behind the body (typically no further than roughly 20° past neutral) and rotating the foot outward beyond approximately 50°. Prone sleeping — lying on the stomach — is the clearest everyday violation, because it simultaneously pushes the hip into extension and external rotation, both of which direct the prosthetic ball toward the now-vulnerable front of the socket.

The sitting rules so central to posterior-approach recovery — raised toilet seats, chairs that keep hips above knee level — are considerably less critical here, because forward flexion is not the hazard. A patient who applied the posterior rule-set after an anterior approach would be assiduously protecting the wrong side of the joint.

Beyond the precaution picture, patients considering the DAA should be aware it carries a distinct intraoperative risk profile: the lateral femoral cutaneous nerve runs close to the surgical corridor, making lateral thigh numbness a recognised complication, and femoral preparation can be technically demanding on a steeper learning curve. These considerations are best explored during pre-operative assessment, where the suitability of each approach for the individual patient can be properly weighed.

Safe sleeping positions after hip replacement

The first night home is when sleeping guidance becomes urgent rather than theoretical. For most patients, the critical questions are the same: which way to lie, what to place between the legs, and how to get into bed without triggering the movements the surgeon warned about.

Posterior-approach patients

Back sleeping is standard for at least six weeks. One or two pillows placed between the knees and ankles serve a mechanical purpose — they hold the operated leg in abduction, which physically prevents the hip from rolling into adduction or internal rotation during sleep. Without that buffer, an unconscious shift in bed can place the joint directly into the forbidden triad at its most unguarded moment.

Sleeping on the non-operative side is permitted in some protocols, provided the pillow buffer stays in place. The pillows are not a comfort measure; they are the mechanism that stops the two legs from closing and the operated hip from drifting inward.

Stomach sleeping is not permitted for either group.

Anterior-approach patients

Because extension and external rotation are the hazard for this group — not flexion — the sleeping concern runs in the opposite direction to posterior-approach patients. Prone sleeping loads precisely those directions and is not permitted during recovery. Lying on the non-operative side is typically the preferred early position.

Both groups are generally advised against sleeping in a recliner long-term. Sustained hip flexion in a reclined seat causes progressive joint stiffening — a separate problem from acute dislocation, but one that can impair full recovery of movement range over weeks.

Getting in and out of bed (posterior approach)

• Entering: lead with the non-operated leg; slide in keeping toes pointing to the ceiling throughout
• Exiting: operated leg leads first, with the same toes-up rule maintained through the roll
• Roll bed covers down before getting in — reaching forward to pull up sheets bends the hip past the permitted range

Sitting height, transfers, and daily movement athome

Low seating is the most common domestic dislocation scenario for posterior-approach patients, and the geometry explains why. Any chair that drops the hips below knee level automatically angles the joint toward the forbidden zone: the combination of deep flexion, even without adduction or rotation, can be enough to lever the prosthetic ball toward the unwalled back of the socket.

The practical fix is simple in principle. Chairs, toilet seats, and bed surfaces should keep the hips at or above knee height throughout the recovery period. Standard adaptive equipment for posterior-approach patients typically includes a raised toilet seat, a firm seat cushion or chair raiser, and armrests sturdy enough to take body weight on rising. These are temporary measures, tied to the 90-day healing window while the posterior capsule and tendons reattach — not permanent features of life after hip replacement.

Transfer technique from a chair (posterior approach)

• Walk to the chair, turn away from the operated side, and feel the chair edge against the backs of the legs
• Slide the operated leg forward before lowering — the foot stays out in front, never tucked back under the seat
• Lower down by pushing through the armrests and the non-operative leg
• To stand, push up through the arms and non-operative side before the hips fully rise; the operated leg leads the movement upward and outward

Anterior-approach patients

Seat height is considerably less critical here, because deep flexion is not the hazard. That said, very low, soft sofas or deep recliners can still cause difficulty: rising from them tends to force the hip into extension and external rotation, which are the movements this group does need to avoid.

Both groups benefit from temporary home adaptations — long-handled dressing aids, grab rails at the toilet and shower, and raised seating throughout the house — that remove the need to make a calculated movement decision every time a routine task arises.

SPAIRE, tendon-sparing surgery, and the shifting evidence on precautions

Preserved tendons change the equation. The SPAIRE technique — Saves Piriformis And (Obturator) Internus with Repair of (Obturator) Externus — uses the same posterior corridor as the standard approach but leaves the short external rotators intact rather than dividing them.

The benefit is mechanical and neurological at once. The intact obturator internus passes directly over the posterior femoral head, creating a passive tether — a biological seatbelt — that resists dislocation during sitting, rolling, and transfers. The preserved Golgi tendon organs and muscle spindles within these tendons also maintain proprioceptive signalling, allowing the joint to 'feel' its position throughout recovery. Conventional posterior-approach patients lose this awareness when the tendons are cut, leaving the hip neurologically blind until the healing period resolves — closely paralleling the 90-day risk window described above.

Clinical evidence suggests SPAIRE patients may face a meaningfully less restrictive precaution burden, though the evidence base is still developing and individual anatomy, implant selection, and surgical technique remain relevant variables.

A broader debate runs alongside this. Crompton et al. (2020) found no statistically significant reduction in dislocation rates from traditional posterior-approach hip precautions. A UK GIRFT analysis of approximately 230,000 primary THAs found a six-month dislocation rate of around 0.8%, with no increase at hospitals that had stopped mandating traditional restrictions; Guo et al. (2024) reached a comparable conclusion.

This evidence is real and has shifted practice at many centres — but population averages do not determine individual outcomes. Patients who do dislocate face serious consequences: the majority re-dislocate, and nearly half require revision surgery within two years. For patients with higher individual risk profiles — younger age, low BMI, significant comorbidities — following movement precautions during the biological healing period remains a clinically defensible position even as the evidence for their universal application has weakened. The prudent reading of the data is not that precautions are unnecessary, but that which precautions are necessary, and for whom, is a question that deserves a more nuanced answer than a blanket six-week restriction sheet.

Determining which approach and which recovery framework suit a given patient requires formal specialist assessment. Prof Paul Lee's hip practice at Hip Replacement Lincolnshire's centres in Sleaford and Grantham accepts patients without a GP referral; appointments can be booked directly at hipreplacementlincolnshire.co.uk.