The human body maintains the sagittal balance of the spinopelvic complex by virtue of its bony morphology and soft tissue tension. When changing position from standing to sitting, the femur rotates from a vertical to a horizontal position, which is approximately 90° in relation to the ground. The upper body is maintained upright. During this movement, the pelvis lies on both hip joints and rotates on the co-axis of the femoral heads, distributing femur movement to the trunk and over the hip joint and spine [5,6,7,8, 15, 16, 20,21,22]. Hip arthroplasty surgeons must identify the spinopelvic movement patterns in their patients, as patients demonstrating greater hip flexion during daily activity may be at greater risk of anterior impingement and posterior dislocation [15]. Our study assessed how the lumbar spine, pelvis, and hip joint move during change from a standing to a sitting position and the influence of lumbar disc degeneration on such spinofemoral movement.
We show that patients with LDD had 5° lesser LL on standing, indicating a relatively kyphotic posture. Blizzard et al. reported that spinal deformities decreased lumbar lordosis [16]. Patients with LDD may be unable to stand erect and may walk with the trunk leaning forwards [23, 24]. Previously, Liang et al. [25] reported that patients with lumbar disc herniation, experiencing sagittal imbalance, had a significantly increased SVA of 11.6 cm, on average. With an increased SVA and decreased LL, patients in the LDD group posteriorly rotated their pelvis by 5° more (PT) in compensation.
When changing from a standing to sitting position, patients were instructed to sit on a standard chair to achieve a horizontally placed femur. Images were acquired using the EOS system to ensure reliability. The pelvis acts as a link between the upper body and lower limbs. In 1992, Duval-Beaupere et al. introduced the concept of PI as a cornerstone for describing spinofemoral relationships [26]. The PI was not significantly different between the groups in our study and did not change from the standing to the sitting position, highlighting its anatomical characteristics.
During sitting, the L1 slope change was relatively small (4.7°). As the kyphotic thoracic spine forms a cage in combination with the ribs and respiratory muscles, a minimal sagittal ROM (0.1°) is present, as reported by Ochi et al. [8] This cage provides a stable base for the cervical spine and head. Below L1, the lumbar spine, pelvis, and femur rotate in a chain-like manner to distribute the flexion of the spinofemoral movement [21]. Spinofemoral flexion is the combination of intrinsic motion of the acetabular-femur joint and extrinsic lumbar spine movements. The total lumbar spinofemoral motion for all patients in our study was 90.9° on an average. For all the patients, the mean lumbar spine flexion was 30.3° and hip flexion was 58.1°. The hip joints bear only-two thirds of spinofemoral movement from standing to sitting. The pelvis rotated 27.4° usually posteriorly (change in SS) in all the patients. Many studies have demonstrated a SS change of 22°–27° from standing to sitting [6,7,8, 15, 20]. It has been reported that every 1° increase in the pelvis posterior tilt increases the acetabular component anteversion by 0.7° [22, 27,28,29,30]. The functional acetabular anteversion increased by 19° (27.4° × 0.7) on average in our patients. The pelvis movement helped to prevent posterior dislocation via a combination of reduced hip movement and greater anteversion change.
Lumbar spine degeneration alters spinopelvic alignment and motion. In our study, patients in the LDD group had 7° lesser total spinofemoral flexion. This was mainly due to decreased movement in the lumbar spine (16°). To compensate for reduced lumbar motion, patients had to recruit greater hip flexion (7°) to place the femur flat when sitting. The spine/hip ratio was significantly lower in the LDD than in the control group (0.3 versus 0.7; p < 0.001). The LDD group had 8° lesser pelvis rotation, which would decrease anteversion changes by 29.6% (8°/27°).
There were more female and elderly patients in the LDD group, and the mean age of patients in this group was higher than in the control group. Multivariate analysis revealed that sex was not a significant predictor of a decreased spine/hip ratio. Moreover, the cadaver and clinical studies have not indicated a significant effect of sex on lumbar disc degeneration [31, 32]. Older patients exhibit more lumbar degenerative changes, including hypertrophy of facets, degeneration of intervertebral disks, and osteophytosis of vertebrae. These phenomena would lead to disc space narrowing, loss of LL, and decreased lumbar spinal ROM. A study of 214 male patients by Burton et al. [32] revealed that reduced lumbar flexibility was multifactorial, and included lumbar disc degeneration and increased age. Furthermore, Schepper et al. reported a correlation between increased frequency of radiographic disc degeneration and age [33]. Arthroplasty surgeons should be aware that older patients with degeneration of multiple lumbar discs have a significantly different lumbar spinopelvic motion pattern.
When patients have a limited ROM in the lumbar spine, the risk of dislocation and need for revision after THA increases markedly./ Based on a 12-month follow-up, Perfetti et al. reported that, compared to the controls, the THA patients with prior spinal fusion were 7 times more likely to dislocate their prostheses (p < 0.01) and 4 times more likely to need revisions (p < 0.01), [10]. Sing et al. reported that the dislocation rate was 2.4% for THA patients without prior spinal fusion, 4.3% for patients with 1 to 2 levels fused, and 7.5% for patients with 3 to 5 levels fused [11]. In this study, patients with previous spine surgery were excluded. Patients with multiple degenerative lumbar discs could be more difficult to identify than those with a clear history of spine fusion surgery. Surgeons should pay particular attention to these patients with poor spinopelvic mobility, as they have greater hip flexion, increasing their risk of impingement and posterior dislocation. As both patients and surgeons are increasingly more prone to relax hip precautions postoperatively, our study outcomes may help surgeons to identify the THA candidates with stiff lumbar spine movements preoperatively. However, methods to identify these patients more easily than by X-ray imaging while standing and sitting need further investigation. Surgeons may have to place the prosthesis in a more individualized position during surgery; this has gained increased attention. It is necessary to preparing special implants, such as a dural-mobility cup, in advance, particularly when patients have additional risk factors for dislocation. Personized postoperative rehabilitation protocols should be prescribed in these cases.
This study had several limitations. First, static standing and sitting images do not fully represent a patient’s pelvic orientation during activities of daily living. Patients usually had their hip dislocated posteriorly in a specific position involving flexion, adduction, and internal rotation. Further research is required to evaluate this issue. Dislocation on sitting was rare, unless the hip was highly unstable. We did not perform assessments in a squatting position as painful hip joints hindered patients from squatting fully prior to surgery. We have been following-up these patients to record any postoperative dislocation and to relate our measurements to clinical outcomes. Second, patients preparing for THA may have stiff and painful hip motion, leading to increased lumbar spine flexion on sitting. There was no difference in the AVN stage between the two groups, and the majority of the patients were of FICAT Stage III AVN; this helped mitigate this bias. Third, the quantitative influence of the severity of lumbar disc degeneration on spinofemoral movement was not included. More patients and longer follow-up are required to analyze these differences.
