Spinopelvic mobility and acetabular component position for total hip arthroplasty

Materials and Methods

This was a prospective study of radiographic outcomes of primary THA according to spinopelvic mobility. These operations were performed between December 2014 and March 2016. Our Institutional Review Board approved the study design, and patients signed an informed consent for release of their data. Clinical data for correlation to radiographic data was limited to gender (females = 75/151 patients (50%), mean age is 62.9 years (27 to 86), mean body mass index is 27.7 (16.9 to 45.3) and the diagnosis was (hips): osteoarthritis: 137, developmental dysplasia of the hip: nine, osteonecrosis: seven, post-traumatic: four, others: three. A total of 151 patients (160 hips) were enroled in the study with no exclusions, and there was complete pre-operative, intra-operative and post-operative data at three to six months. Surgery was performed through a mini posterior approach^10,11 by a single surgeon (LDD) with the use of computer navigation for acetabular component centre of rotation, inclination and anteversion (ORTHOsoft, Zimmer Biomet, Montreal, Canada). Acetabular component angles were adjusted by the software used for the navigation to the radiographic plane of Murray.^11,12 Femoral preparation was performed first so the acetabular component anteversion would achieve combined anteversion in its safe zone.^13,14 Patients were discharged the same or following day, full weight-bearing without precautions, but told they could bend as necessary to do shoes and socks as long as they did so between their knees (with the legs abducted). Walking was their only physiotherapy.

Pre-operatively, radiographs obtained were a low anteroposterior (AP) pelvis with proximal femur with the beam centered on the symphysis pubis, an iliac oblique Judet view including the femur was used as the lateral view, and a lateral spine-pelvis-hip-proximal femur in the standing and sitting position (Fig 1). All radiographs were made by the same radiographers. For the standing lateral spinopelvic-proximal femoral film, the patient’s left hip was placed adjacent to the cassette with their arms resting at 90^o on a support regardless whether the right or left hip was of interest. The x-ray beam was centred at the greater trochanter perpendicular to the patient’s axial line, and the source-film distance was 183 cm. The same radiographic criteria was then used with the patient sitting on a stool with his or her back straight and the angle between the thighs and trunk was approximately 100^o, not 90^o, because better visualisation of the pubic symphysis was possible with that angle, and it was the comfortable sitting position. Post-operatively, the radiographs were repeated at the first clinic visit which was between six weeks and three months. A total of 84 patients (84 hips) had radiographs repeated at the six month to one year follow-up. From the pre-operative AP pelvis we obtained the hip length and offset; from the post-operative AP pelvis we obtained hip length, offset and acetabular component inclination and anteversion.^15-17 Hip length and offset were compared with the opposite hip if it was not diseased.

From the pre-operative and post-operative lateral spinopelvic radiographs, the skeletal measurements of pelvic incidence, sacral tilt (ST) (also named sacral slope) and pelvic femoral angle were obtained using PACS Synapse by two observers (HI, RM) (Fig 1). From the post-operative film the implant measurements of ante-inclination and sacral acetabular angle were also measured (Figs 1 and 2). Pelvic incidence is a static measurement so is the same for both standing and sitting (our normal = 42^o to 64^o), and is the measure of the width of the pelvis so that as pelvic incidence increases from low to high values the lordosis of the spine increases, and the femoral head moves further anteriorly.⁸ We established normals from our data combined with those in the literature.^5,8,18 ST is a dynamic measure of pelvic tilt from the anteriorly tilted standing position (normal = 40^o +/- 10^o) to posterior tilt with sitting (normal = 20^o +/- 9^o). ∆ST is the difference between the standing and sitting ST (normal = 11^o to 29^o). Pelvic femoral angle is the measure of femoral extension standing (normal = 180^o +/- 10^o) and of flexion sitting (normal = 132^o +/- 12^o).⁸ From the post-operative films the implant measurements of ante-inclination (standing normal 35^o +/- 10^o, sitting 52^o +/- 11^o) and sacral acetabular angle (75^o +/- 15^o) were also measured (Figs 1 and 2).

Figs. 2a - 2b

a) Post-operative standing lateral spinopelvic-hip radiograph of hypermobile flex construct: pelvic incidence (PI) 66^o (high PI). Sacral tilt (ST) 55^o (high). Intra-operative computer navigation acetabular component positions: inclination 35^o, anteversion 21^o, combined anteversion 33^o results in normal sagittal component positions: ante-inclination (AI) 30^o, sacral acetabular angle (SAA) 85^o, pelvic femoral angle (PFA) 181^o. b) Post-operative sitting lateral spinopelvic-hip of hypermobile-flex construct: ST 22^o so ∆ST 33^o (high). AI 63^o; SAA 85^o; PFA 126^o (all normal for hypermobile patients).

Spinopelvic mobility is classified as normal, stiff or hypermobile according to the movement measured by the change in ∆ST between standing and sitting (Fig. 1).² We consider hypermobility of the spine, not caused by kyphosis of the spine, as a variant of normal with greater mobility (∆ST > 30^o). In patients with hypermobility, their normals are all increased by 10^o. There are some patients who have hypermobility because of severe flattening and kyphosis of the spine with sitting and an absolute sitting ST < 10^o. These patients are pathologic and this is not normal hypermobility. Conversely, stiff hips with spinal imbalance have a difference between standing and sitting ∆ST of < 10^o.

We determined the appropriate acetabular component position according to the spinopelvic mobility from our earlier study.² For normal hips, and kyphotic hips with normal mobility, the inclination was 40^o and anteversion 20^o with combined anteversion of 25^o to 45^o. For hypermobile normal hips, and hypermobile kyphotic hips, lesser inclination of 35^o to 40^o and lesser anteversion of 15^o to 20^o is necessary to prevent excessive verticality of the acetabular component with sitting. Stiff hips needed inclination near 45^o, and anteversion of 20^o to 25^o with combined anteversion of 35^o to 50^o, to open the orientation of acetabulum to compensate for loss of the pelvic movement. Centre of rotation was measured intra-operatively from the computer and hip length and offset from post-operative radiographs.¹⁴

Data collection tested whether the acetabular component positions used resulted in ante-inclination and sacral acetabular angle in the normal range. Abnormal ante-inclination or sacral acetabular angle did not always have the same consequence so severity of imbalance was categorised as pathological, dangerous or inconsequential. Pathological in this setting was defined when even ideal acetabular component position was obtained it did not overcome the spinal imbalance and a high risk remained for impingement; dangerous imbalance meant that correct acetabular component position would result in ante-inclination and sacral acetabular angle in the normal range, but precision of the component position is required; and inconsequential imbalance meant there is an abnormal measurement, usually of one value, which by itself is not a risk so was clinically irrelevant.

Results

The inter-observer reliability for this radiographic outcome study was 0.993 (95% confidence interval 0.926 to 0.969) which is considered excellent agreement. In addition, 73 of 84 patients (84 hips) with six months to one year follow-up had no change in their measurements, nine improved and two became worse. Therefore, the six-week radiographs provide a reliable picture of the patients’ outcome.

We identified five patterns of spinopelvic mobility: normal (Fig. 1), hypermobile variant of normal (Fig. 2), and three patterns of spinal imbalance: hips maintained in anterior tilt (stuck standing, Fig. 3); hips held in posterior tilt (stuck sitting, Fig. 4); and kyphotic (Fig. 5). Hypermobile hips have pelvic mobility of more than 30^o between standing and sitting; stuck standing hips mean that the pelvis is fixed in anterior tilt, and with sitting does not shift posteriorly beyond an absolute ST of 30^o; stuck sitting means that the pelvis is stuck in posterior tilt with standing, and the ST does not tilt anteriorly beyond 30^o. If hips that are stuck standing or stuck sitting are stiff, the change of ST between standing and sitting is ≤ 10^o (∆ST < 10^o). Hips that were so stiff that the ∆ST was ≤ 5^o were those that had biological or surgical fusion and thus were termed fused hips. Hips that had fusion also had a stiff acetabulum so that the ante-inclination between standing and sitting was < 5^o.

Figs. 3a - 3b

a) Standing lateral spinopelvic-hip radiograph fixed with anterior tilt (stuck standing). Pelvic incidence (PI) 73^o (high PI); sacral tilt (ST) 44^o; sacral acetabular angle (SAA) 96^o; ante-inclination (AI) 52^o; pelvic femoral angle (PFA) 201^o (SAA and PFA are abnormally high). b) Sitting lateral spinopelvic-hip radiograph fixed with anterior tilt: ST 39^o so ∆ST 5^o (biological fusion). AI 57^o; PFA 122^o (both normal). At surgery the acetabular component was placed at 42^o inclination, 20^o anteversion with combined anteversion of 40^o. With these acetabular component numbers the post-operative AI and SAA were normal for both standing and sitting.

Figs. 4a - 4b

a) Post-operative standing lateral spinopelvic-hip radiograph of construct fixed in posterior tilt and stiff: Pelvic incidence (PI) 35^o (low PI); sacral tilt (ST) 27^o which means this structure is stuck sitting. Computer navigation intra-operative component position: inclination 45^o, anteversion 21^o and combined anteversion 33^o results in normal sagittal implant position: ante-inclination (AI) 38^o, sacral acetabular angle (SAA) 65^o and pelvic femoral angle (PFA) 186^o. b) Post-operative sitting lateral spinopelvic-hip radiograph of construct fixed in posterior tilt: ST 24^o so ∆ST 3^o (fusion). AI 41^o so ∆AI 3^o (stiff acetabulum). PFA 109^o which is more flexion than normal because all movement must occur at the hip. This hip remains at risk for impingement even with correct acetabular component angles.

Figs. 5a - 5b

a) Post-operative anterior-posterior radiograph showing a dislocation. Intra-operative computer navigation component position: inclination 38^o, anteversion 23^o, combined anteversion 33^o. This component position gave normal skeletal and sagittal acetabular component positions on the post-operative lateral spinopelvic radiograph. b) Pre-operative sitting spinopelvic hip radiograph of same patient shows kyphotic spine position. The sacral tilt 0^o.

Discussion

This study was conducted to understand the consequence of spinal disease, primarily degenerative disc disease and arthritic change of the spine with ageing,²¹ on the optimal acetabular component position with primary THA. The first question we asked was if we could control the angular change of the acetabular component, which occurs between standing (ante-inclination), by the intra-operative acetabular component inclination and anteversion. If we could not, could we identify those hips that remained at high risk for impingement. We found that with the targeted acetabular component positions we used we could keep ante-inclination and sacral acetabular angle in their normal range in 145/160 hips (90%). In all, six of the 15 hips had risk for impingement because we did not achieve the desired acetabular component position and nine hips were at risk because of spinal imbalance. A total of seven hips that had pathological imbalance also had biological or surgical fusion, and it is these hips which were at highest risk for impingement. These hips were at risk because restricted pelvic movement does not allow the acetabulum to open during flexion of the hip with sitting. So the acetabular component must be mechanically opened with high inclination and anteversion angles. If inclination exceeds 45^o, there is a trade-off of potentially increasing wear²² so the surgeon must balance the risks of dislocation with the age and activity level of the patient. With higher angles of inclination and anteversion, the ante-inclination and sacral acetabular angles were in the normal range for patients with stiff hips (stuck standing or stuck sitting). However, even with these high acetabular component angles, seven fused hips remained at risk for impingement at the extremes of movement because the acetabular opening between standing and sitting is stiff. The high component angles protected the patients in this study from dislocation, but we have observed this correlation in patients with late dislocation.

To correctly position the acetabular component at surgery, one must recognise the five patterns of spinal movement. Two are normal with one of these being hypermobile, and these have almost no risk for impingement if the biomechanical reconstruction is appropriate. Pre-operatively, 87 of 160 hips (54%) had normal spinopelvic mobility and this is similar to previous findings.²¹ Post-operatively, 125 hips (80%) had normal spinal mobility so even with degenerative disc disease the stiff spinal mobility can be converted to normal spinopelvic movement after release of hip contractures and stiffness by THA. It is important to recognise that the acetabular component positions targeted for hypermobile hips are lower than normal (Table II), and for stiff hips are higher than normal (Table III).

Hips with a kyphotic spine present the unique problem that when seated the pelvis is tilted as far posteriorly as possible so with any increased flexion, such as knee to chest movement or deep squats, all the movement occurs at the hip which means either bony or component impingement can occur, and posterior dislocation can be the result. Hypermobile kyphotic hips are the ones that have ‘drop out’ dislocation if the acetabular component inclination and anteversion are high because with sitting the sagittal acetabular component position is vertical (> 75^o), and the jump distance of the femoral head can be exceeded. Kyphotic hips with normal mobility should have the same inclination and anteversion as normal hips; kyphotic hips which have the pelvis fixed in posterior tilt (stuck sitting) need high inclination and anteversion to create a mechanically opened acetabular component for sitting, but this creates the risk of a vertical ante-inclination so we considered these hips to have pathological imbalance and would consider the use of dual mobility articulation.

The critical limitation of this study is that, as a radiographic study of the relationship of intra-operative acetabular component position to spinopelvic mobility, we have only surrogate measurements for impingement, and no clinical episodes which we can measure. In this study we do not have evidence of a correlation between these outliers and late dislocation, but we have observed this in an ongoing study of acute and late dislocations. The validation of our data for primary THA will be possible when the relationship between proven impingement, dislocation, pain and loosening, and outliers of acetabular component ante-inclination and sacral acetabular angle are reported. A second method to validate these data will be by study of dynamic movement of hip replacements to confirm our study of static radiographs. We have initiated this investigation.

There is a recent study of ceramic-on-ceramic articulations that squeak, that showed the anteversion of the acetabular component on sagittal spinopelvic radiographs revealed the cause of edge-loading.²³ Their study, although using a different flexion position than we did, confirms our finding that the sagittal component position is more informative of functional component position than that on the supine AP pelvis film. Where instability has occurred after THA, it would be beneficial to examine sagittal spinopelvic-hip radiographs to determine if the criteria for impingement is present. There are studies using dual mobility articulation in all patients to manage the problem of spinal imbalance.²⁴ Our data show that with high acetabular component angles even the hips with fused spines can have an open acetabular component, and we had no complications in the patients operated with conventional articulations. However, the patients with pathological imbalance, fused and flat kyphotic, are those in whom we may choose to use more constraint.

In summary, we found that correct intra-operative acetabular component inclination and anteversion by the surgeon will compensate for most spinal imbalance. Pre-operatively there were 24 hips with dangerous imbalance and 18 with pathological imbalance so 26.2% of our population (42/160 hips) required precise acetabular component implantation. With precision, 22 of 24 hips with dangerous imbalance, and 11 of 18 with pathologic imbalance, had normal post-operative ante-inclination and sacral acetabular angles. In seven hips with pathological imbalance the acetabular mobility remained stiff (∆ ante-inclination < 5°) so that the risk for impingement remained high. These are the hips that may be candidates for increased constraint such as dual mobility articulation.

Take home message: Spinopelvic mobility and component position of total hip arthroplasty are related, and patients with spinopelvic imbalance, as well as patients with component malposition, are at risk for impingement.