Spine-hip relations in patients with hip osteoarthritis.

Patients with hip osteoarthritis often have an abnormal spine-hip relation (SHR), meaning the presence of a clinically deleterious spine-hip and/or hip-spine syndrome.Definition of the individual SHR is ideally done using the EOS® imaging system or, if not available, with conventional lumbopelvic lateral radiographs.By pre-operatively screening patients with abnormal SHR, it is possible to refine total hip replacement (THR) surgical planning, which may improve outcomes.An important component of the concept of kinematically aligned total hip arthroplasty (KA THA) consists of defining the optimal acetabular cup design and orientation based on the assessment of an individual's SHR, and use of the transverse acetabular ligament to adjust the cup positioning.The Bordeaux classification might advance the understanding of SHR and hopefully help improve THR outcomes. Cite this article: EFORT Open Rev 2018;3:39-44. DOI: 10.1302/2058-5241.3.170020.

Introduction

The concept of spine-hip relation (SHR) defines the interaction between the lumbopelvic complex (LPC) and the hip joint. As the pelvis is a common anatomical structure for these two joints, changes in one joint can potentially clinically impact the other. This pathological situation is defined as the spine-hip syndrome (SHS)[1] or hip-spine syndrome (HSS),[2,3] depending on whether the primary pathological structure is the spine or the hip, respectively. This instructional review aims to clarify and classify the SHRs that are commonly encountered in patients with hip osteoarthritis (OA).

Spino-pelvic parameters

In order to understand and define the SHR, it is important to understand the significance of the spino-pelvic parameters: sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL) and the C7 plumb line (C7pl) (Fig. 1).[4,5] SS, PT and LL are ‘functional parameters’, as their value varies with body position; PI is a ‘morphological parameter’ as it is constant for an individual; SS, PT and PI are linked by the equation PI = SS + PT.[4,5] Only in health are the PI and LL usually linked (LL = 0.54 PI + 27.6) and the PI can be considered as a biomechanical marker to estimate the potential sagittal pelvic range of motion (ROM).[6] Definition of an individual’s sagittal balance through the assessment of these spino-pelvic parameters on standing spino-pelvic lateral images is best achieved with the EOS® imaging system (EOS Imaging, Biospace®, Paris, France), or alternatively with conventional radiographs.[1,7] The comparison of those parameters between sitting and standing lumbopelvic lateral images gives an estimate of the pelvic sagittal ROM when passing from standing to sitting position (Fig. 2).[2]

Fig. 1

The main spino-pelvic parameters: the sacral slope (green); the pelvic tilt (red); the pelvic incidence (blue).

Fig. 2

The ways in which an abnormal SHR can contribute to femoro-acetabular impingement (red) due to abnormal lumbopelvic kinematics (SHS type 1, top left) or a low pelvic incidence (top right). With healthy interaction between the flexible LPC and flexible hips, femoro-acetabular impingement is less likely to occur (bottom right).

Abnormal SHR

A harmonious (normal) SHR occurs when a healthy flexible LPC interacts with healthy flexible hips. As soon as one of these anatomical structures becomes stiff, a clinically deleterious compensatory mechanism is often initiated by the other one. This is termed either ‘spine-hip syndrome’ (SHS)[1] or ‘hip-spine syndrome’ (HSS),[2,3] depending on whether stiffness begins first in the lumbopelvic complex or the hip(s), respectively.

Spine-hip syndrome

The two main causes of SHS should be differentiated. One cause is ageing of the spine, which is mainly seen in the elderly, with the pelvis becoming progressively stiff and retroverted (abnormal lumbopelvic kinematics or SHS type 2).[8] The other involves muscular trunk imbalance, also previously described as ‘lower crossed syndrome’, and is mainly seen in young adult patients and where the pelvis displays insufficient retroversion when sitting (abnormal lumbopelvic kinematics or SHS type 1).[9-12]

Ageing of the spine, resulting mainly from degenerative disc disease and osteoporotic vertebral collapse, might affect a patient’s sagittal balance in severe cases (Fig. 3). This degenerative process causes progressive loss of lumbar lordosis and increased pelvic retroversion (SS decreases and PT increases), therefore reducing the lumbopelvic complex curvature and flexibility. In the early stages, the patient compensates for this abnormal pelvic retroversion, in the standing position, by permanently extending the hips (‘compensated stage’ with normal C7pl). However, with progressive ageing of the spine, this compensatory mechanism becomes ineffective and the patient starts to become sagittally imbalanced as shown by the excessively anterior C7pl (‘decompensated stage’).[8,13] A sagittal imbalance substantially affects the quality of life and can be clinically deleterious[14-16] because, when standing, the patient has to hyperextend the neck in order to keep a horizontal sightline, and the hips are at risk of OA secondary to aberrant standing functional acetabular orientation, under-coverage of the femoral head anteriorly and posterior impingement.[17,18] With THA, implants would therefore be at a higher risk of failure, notably through complications related to edge-loading and anterior prosthetic instability.[1,19-26] This SHS type 2 appears to be frequent (26% to 40%) in patients scheduled for primary THA.[8,18]

Fig. 3

Type 2 SHS resulting from ageing (from left to right) of the spine. C7 plumb line in yellow.

Muscular trunk imbalance can lead to paradoxically poor pelvic retroversion when a patient sits or squats, thus leading to an aberrant sitting functional acetabular orientation responsible for femoral head over-coverage and anterior impingement (Fig. 2).[27-29] This would probably predispose to hip OA in native hips, and posterior instability and posterior edge-loading in prosthetic hips. The origin of these abnormal lumbopelvic kinematics is unknown. The diagnosis is made on lumbopelvic lateral images showing < 10° decrease in pelvic retroversion[30] between standing and sitting positions (Fig. 2).

In addition to the aforementioned pathological types of lumbopelvic sagittal kinematics, variation in PI can also affect THR outcomes in patients with a very small PI (< 35°) by increasing the risk of edge-loading and instability (Fig. 2).[31] Patients with a very small PI are considered to have a ‘patho-anatomy’ and are likely to have a small sagittal pelvic ROM when sitting or squatting, therefore making them use more of each hip’s cone of mobility (hip users, Fig. 4).[10,12] In contrast, patients with a large PI and sagittal pelvic ROM would use less of each hip’s cone of mobility (spine users, Fig. 4). A classification of SHRs based on the assessment of the individual PI and SHS has recently been published under the Bordeaux classification (Fig. 5).[10,12]

Fig. 4

Relationship between spine and kinematics. Note the larger hip cone of mobility (demonstrated with the larger sacro-femoral angle in orange) of a ‘hip user’, compared to a ‘spine user’ with a larger pelvic ROM but a smaller hip cone of mobility.

Fig. 5

The Bordeaux classification of SHRs.

Hip-spine syndrome

Hip OA is known to affect hip mobility/flexibility secondary to the generation of articular impingement through osteophytosis and/or contracture of the capsule. The lumbopelvic complex will therefore in return compensate for this hip stiffness with increasing lumbar lordosis of the spine (abnormal standing functional lordosis) in cases of fixed flexion deformity of the hip, in order to allow the patient to stand upright. This lumbopelvic compensation is likely to be clinically deleterious by causing degeneration of the spine and its associated complications (low back pain, etc.).[32] HSS can be clinically tested with physical examination by performing Thomas’ test to compare both hips (the degenerated and the healthy contralateral) and a radiographic ‘extension test’ with EOS® imaging to measure the sacro-femoral angles (Fig. 4).[33] The discrepancy between degenerated and healthy hips enables the surgeon to discriminate between SHS and HSS; that is to say that both sides are likely to be affected in type 2 SHS but only one leg is likely to be affected in HSS due to unilateral OA.[30]

As illustrated in Figure 5, hip OA patients can display various SHRs that we have classified as normal or abnormal, HSS or SHS, or a combination of the two. The Bordeaux classification, aiming at classifying the SHRs with SHS has recently been published.[10,12]

Clinical implications

SHS is a cause of hip OA, which in turn can aggravate the spine by way of a HSS. SHS and HSS are frequently seen in patients awaiting THA,[8,18] and pre-operatively screening patients with abnormal SHR could refine the surgical planning and therefore THA outcomes. In patients with HSS, anterior capsule release could provide back pain relief and help with better hip balancing and leg length adjustment. SHS patients, by contrast, may benefit from corrective spinal surgery before the hip replacement.[34-36] Patients with SHS usually present with a stiffer LPC, which makes those patients mainly ‘hip users’ in situations of aberrant functional acetabular orientation,[10,12] increasing the risk of edge-loading and articular impingement (on a native or prosthetic hip). In order to face this aberrant functional acetabular orientation and prevent those complications happening, it seems reasonable to first use a higher tolerance implant (larger diameter head, dual mobility)[25] and second, to determine a personalised cup orientation.[10,17,37,38] This relates to the concept of kinematically aligned total hip arthroplasty (KA THA), which has recently been promoted by Riviere et al.[10] In contrast with the attractive but costly optimized positioning system® (Corin, Cirencester, UK),[39,40] the authors suggest a simple, low-cost and fast way to achieve patient-specific implant positioning by defining the optimal acetabular cup design and orientation based on the individual SHR.[10] In order to improve the positioning of the cup, the surgeon must first pre-operatively define an optimal cup orientation[7,11,18,41,42] based on the assessment of the individual dynamic functional acetabular orientation,[36,37] and second, intra-operatively precisely position the acetabular cup, ideally by simply using the transverse acetabular ligament (TAL). Aligning the cup with the TAL has been shown to be clinically safe and effective[43] and to lead to better standing cup orientation,[44] even in cases of hip dysplasia.[45] If needed, the TAL is also a relevant anatomical landmark with which to adjust the cup anteversion as it enables reproduction of both the anatomical and functional acetabular anteversion of an individual. This would therefore restore the individual acetabular cone of mobility and, as modern hip implants have a head-neck ratio higher than the native hip, would probably prevent articular impingement.[25] Regarding the cup inclination, a surgeon can adjust this by fine-tuning the position of the cup with the TAL or the length of the uncovered roof part of the cup.

Conclusions

Patients with hip OA often have an abnormal SHR, meaning the presence of clinically deleterious SHS and/or HSS. In order to refine surgical planning in THA, it is important to define the individual SHR by ideally using the EOS imaging system® or, alternatively, conventional spino-pelvic lateral radiographs. This would enable screening for patients with abnormal SHR (SHS and/or HSS) for which intra-operative solutions exist. The concept of KA THA consists of defining the optimal acetabular cup design and orientation based on the assessment of the individual SHR/pelvic kinematics, and using the TAL to adjust cup positioning. The Bordeaux classification might improve the understanding of SHR and hopefully help in improving THA outcomes.