The Hip-Spine Relationship in Total Hip Arthroplasty

The core concept. The hip, pelvis and lumbar spine move as one coordinated lumbopelvic complex, and the pelvis is the hinge: its sagittal tilt sets acetabular (cup) orientation and lumbar lordosis at the same time. When normal pelvic mobility is lost, through lumbar degeneration, deformity or fusion, the dynamic protection that keeps a total hip stable through the sit-to-stand arc is lost with it, and dislocation risk rises sharply.

The safe zone and its limits. For most of the history of THA, cup position was judged against a fixed supine "safe zone": the Lewinnek safe zone, classically 40° ± 10° inclination and 15° ± 10° anteversion, and the name an examiner will use when asking this question. The hip-spine concept reframes it. Acetabular orientation is dynamic, changing with posture as the pelvis tilts, so a cup measured inside Lewinnek's zone on a supine film can be functionally malorientated in a patient who cannot tilt. That is the whole argument of this topic, and it is why a large share of dislocating cups are found to be "well positioned".

The normal protective mechanism. Standing to sitting, the healthy pelvis rotates posteriorly and the sacral slope falls, which automatically increases functional acetabular anteversion and inclination. The socket opens to accommodate the flexing femur, and posterior impingement and dislocation are prevented.

Why the stiff spine fails. With a fused or degenerate stiff lumbar spine the pelvis cannot tilt, so the femur flexes against a socket that has not opened up: posterior edge-loading and dislocation. A cup in a textbook supine safe zone can be functionally unsafe in this patient. A growing, ageing population with lumbar fusions has made impaired spinopelvic mobility one of the leading reasons a technically well-positioned THA still dislocates, and recognising and planning for it pre-operatively is now core adult-reconstruction practice.

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The three parameters. All three are measured on a lateral spinopelvic radiograph and are related by one equation: PI = SS + PT.

• Pelvic incidence (PI): the angle between a line perpendicular to the sacral endplate and a line to the femoral head centre. A fixed, morphological parameter that does not change with posture and defines the patient's pelvic "shape".
• Sacral slope (SS): the angle of the sacral endplate to the horizontal. Positional; it falls when sitting as the pelvis tilts back.
• Pelvic tilt (PT): the tilt of the pelvis relative to the femoral heads. Positional; it increases (more posterior tilt) when sitting.

As posture changes, PI stays constant while SS and PT trade off against each other.

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Lumbar lordosis and mismatch. Lumbar lordosis (LL) is normally matched to PI to keep the trunk balanced. When lordosis is lost, through degeneration, flatback or a fusion in poor alignment, PI minus LL exceeds 10°: a key marker of sagittal imbalance and a flag for the high-risk hip-spine patient. The body compensates for the lost lordosis by retroverting the pelvis when standing, which reduces standing functional anteversion and predisposes to posterior impingement.

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Spinopelvic mobility. Mobility is the change in sacral slope from standing to seated. A change of less than 10° defines a stiff spine: the pelvis is not moving. A stiff spine removes the automatic posture-driven adjustment of cup orientation, so a single fixed cup position cannot be safe across both postures.

How much the cup moves. Pelvic tilt drives functional cup orientation in a near-linear way: roughly each 1° of posterior pelvic tilt adds about 0.7–0.8° of functional acetabular anteversion, and reduces functional inclination by a smaller amount. Sitting (posterior tilt) therefore functionally anteverts and opens the cup, while standing or extension (relative anterior tilt) functionally retroverts it. A pelvis stuck in relative anterior tilt, the stiff un-tilting spine, is functionally under-anteverted even when the supine film looks perfect.

How dislocation actually happens. Dislocation is the end of an impingement cascade. At the extreme of motion the femoral neck or component contacts either the liner or rim (prosthetic impingement) or residual bone or osteophyte (bony impingement), and that contact point becomes a fulcrum that levers the head out of the socket. Flexion-adduction-internal rotation against an under-opened cup gives a posterior dislocation; extension-external rotation against an over-anteverted cup gives an anterior one.

The Hip-Spine Classification (Vigdorchik, 2021 Otto Aufranc Award). A practical system that crosses spinal alignment (normal versus flatback) with spinal mobility (mobile versus stiff) to guide component choice. Each category dictates a patient-specific cup position, and dual-mobility components are directed to the highest-risk groups: all of group 2B, and every patient with more than three fused levels. Applied prospectively, this achieved a 0.8% dislocation rate (99.2% survivorship free of dislocation at five years).

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Functional classifications. Other systems, for example the Bordeaux Classification, stratify patients by their functional spinopelvic kinematics (normal, stiff or hypermobile) and by sagittal balance. The common principle across all of them is the same: identify the patient who cannot tilt the pelvis and treat them differently.

Pre-operative assessment. The history asks about previous lumbar fusion and significant back stiffness or deformity. The imaging workhorse is standing and seated lateral spinopelvic radiographs: a single standing lateral provides most of the screening information, and the seated film is added to measure mobility, the change in sacral slope. From the two films derive:

• PI, SS, PT and LL
• PI-LL mismatch
• Standing-to-sitting change in SS (mobility)
• The combined sagittal index, where used

Abandon the universal safe zone. The central practical message is that there is no single cup orientation safe for everyone. Cup inclination and version, and sometimes stem version, must be set to the patient's functional spinopelvic mechanics, while still respecting reasonable coronal targets of approximately 40°/20° ± 10° inclination and version. The plan is screen, classify, set a functional cup target, then choose a forgiving bearing; the low-cost, high-yield step available anywhere is the history for fusion plus two functional radiographs.

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The functional cup target. Adjust target version and inclination from the standing and sitting parameters rather than a fixed safe zone. The combined sagittal index (CSI) integrates spinopelvic tilt with cup orientation, and a standing CSI roughly within 205–245° is associated with stability: too high links to anterior instability, too low to posterior instability. Where the cup alone cannot be made safe, adjust femoral version to optimise the combined (cup plus stem) version. Robotic or navigated systems can execute an individualised functional target precisely.

The protective levers. All of them enlarge the impingement-free arc and the jump distance, the lateral translation needed to dislocate, which rises with head diameter and adequate cup coverage:

• A higher head-to-neck ratio and skirtless heads
• A larger femoral head, which increases jump distance and impingement-free range
• Above all the dual-mobility bearing, a large effective head
• Removing impinging osteophytes intra-operatively, which matters as much as the implant

Dual mobility. The principal solution for the high-risk hip-spine patient. A small head inside a large liner gives a large effective head and increased jump distance, which markedly increase the impingement-free arc and reduce dislocation. It is directed at stiff spines, fusions, deformity, neuromuscular disease and femoral-neck-fracture THA.

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Constrained liners. Reserved for selected salvage or abductor-deficient cases: they transfer stress to the fixation and have their own failure modes.

Who gets into trouble. The patients at risk are those who have lost pelvic mobility. The strongest spinopelvic risk factors for dislocation are a stiff spine, a degenerate or deformed lumbar spine, and prior lumbar fusion, and the magnitude of these risks exceeds that of many traditional patient and surgical factors.

Spinal fusion is the red flag. A systematic review of over one million THAs found prior spinal fusion increased dislocation risk with odds ratios from roughly 1.6 up to about 24 (relative risk around 3). The risk escalates with more fused levels and with fusions extending to the sacrum, because each removes more compensatory pelvic motion. Always ask about, and look for, a lumbar fusion before THA.

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Two failure patterns. The mechanism is loss of the protective posterior pelvic tilt on sitting, and the direction of dislocation is the posture the pelvis is stuck in:

• Stuck standing (pelvis fixed in relative anterior tilt, low functional anteversion): the femur flexes against an under-anteverted socket and the hip dislocates posteriorly.
• Stuck sitting (pelvis fixed in posterior tilt, high functional anteversion): on standing or extension the femoral neck impinges anteriorly and the hip dislocates anteriorly.

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The hypermobile spine. Stiffness is only one end of the mobility spectrum. The opposite problem is an excessive standing-to-sitting sacral-slope change, the Bordeaux "hypermobile" group. Importantly, the stiff arthritic hip itself drives compensatory spine and pelvis hypermobility (an arthritic hip nearly doubles the change in pelvic tilt), and these spinopelvic characteristics tend to normalise after THA once hip flexion is restored, so a pre-operatively "hypermobile" pattern may simply reflect the diseased hip. A genuinely hypermobile spine over-tilts and can produce excessive functional anteversion on sitting and anterior instability.

Mobility changes over a lifetime. Spinopelvic mechanics are not fixed for life. A spine classed mobile (1A) at the time of THA can stiffen later with progressive degeneration or a subsequent lumbar fusion, converting the patient into a higher-risk category and contributing to late dislocation years after a stable index arthroplasty.

Sequencing THA and spinal surgery. When a patient needs both a THA and a lumbar fusion or deformity correction, spinal realignment changes the spinopelvic parameters, so ideally the cup is planned against the final spinal alignment. A THA implanted before a later fusion was targeted to the pre-fusion mechanics and is at risk once the pelvis stiffens; if the spinal deformity is to be corrected, doing or finalising that first, or at least anticipating it, lets the hip be planned to the definitive posture.

The hip-spine relationship has moved internationally from a niche concept to mainstream pre-operative planning over the last decade, driven by an ageing population with lumbar degeneration and the rising number of lumbar fusions. The unifying message across European (EFORT) and North American (Otto Aufranc / Hospital for Special Surgery) reviews is consistent: screen for impaired spinopelvic mobility, abandon the universal safe zone, and individualise component choice.

National joint registries track dislocation and revision-for-instability, and the use of dual-mobility constructs in primary THA has risen, particularly for high-risk and femoral-neck-fracture patients (long-term primary-DM data are still maturing). Across resource settings the low-cost, high-yield intervention is the same: a careful history for spinal fusion and two functional (standing and seated) radiographs — available almost anywhere — to identify the patient who needs an individualised plan.