Pelvis is called pelvic vertebra. The upper part of the pelvis forms the base of the spine via the sacrum, and the lower part connects the lower limbs through the hip joint. It is an important structure for the connection between the spine and the lower limbs. The dysfunction caused by spine and hip joint diseases often leads to the rotation of the pelvis. For patients with THA, the pelvis is the carrier of acetabular prosthesis, which also rotates along with the rotation of pelvis. Therefore, the situation of spine-pelvis-hip of patients with THA needs to be paid attention to, so as to avoid joint dislocation and reduce the edge loading [7-10].
- Findings of previous studies regarding the sagittal rotation of pelvis before and after THA
According to Kamileyvazov et al., there was no difference between the preoperative pelvic spinal sagittal parameters in 28 cases of hip arthropathy with spinal deformity and those of postoperation. However, the patients' posture was significantly improved, and the low back pain was also significantly relieved. Weng et al. divided the patients with hip joint disease into two groups with or without low back pain, and found significant difference in pelvic spinal sagittal parameter of the patients. Digioia et al. reported the preoperative and postoperative pelvic tilt of 84 patients with THA: the average preoperative pelvic tilt in standing position was 1.2 ± 7.9°; the mean postoperative pelvic tilt was 1.1 ± 8.2°. Although there was no statistical significance in the overall change of pelvic tilt, the authors pointed out that there were some individuals whose pelvic tilt was more than 10 degrees. Parrate et al.used gait analysis method to study the pelvic tilt of 21 patients before and after operation, and found that the change scope of pelvic tilt of higher than 37% patients before and 1 year after operation was more than 5 degrees, that of 11% patients was more than 10 degrees, and more than 20 degrees in some individuals. A 5-year follow-up study by Suzuki et al. of 77 patients with DDH who had undergone unilateral total hip arthroplasty after surgery showed that compared with preoperation, the pelvis tilted backward after THA; the average pelvic tilt was 8-20 degrees; and the maximum tilt angle was 25 degrees. The results of these studies were different, which may be related to the different samples. If patients are not divided into groups reasonably, it has little value for clinical guidance.
- Grouping of patients
As Phan et al reported, patients may fall into four categories according to the relationship between spine-pelvis mobility and sagittal plane balance: first, good mobility (flexibility) and balance; second, poor mobility (stiffness) but balance; third, good mobility (flexibility) but imbalance; fourth, poor mobility (stiffness) and imbalance. They also applied the categorization to guide the angle of acetabular prosthesis installation. This method focused on the rotation of the pelvis from standing position to sitting position. Hironori et al. by dividing patients with hip arthropathy into three categories: preoperative sagittal balance-postoperative balance group, preoperative sagittal imbalance-postoperative balance group,and preoperative sagittal imbalance-postoperative imbalance group, pointed out that preoperative sagittal imbalance-postoperative imbalance group had poor outcome. This classification method focused on the effect of spinal pelvic sagittal balance on surgical outcomes.
Both literature reports and our own clinical findings, pelvic sagittal rotation is affected by spine and hip joint. Therefore, classification of patients based on spinal function and hip joint flexion contracture, and rules of pelvic sagittal rotation before and after operation would provide better reference for preoperative plan. Severe hip osteoarthritis reportedly increases lumbar lordosis , pelvic anteversion, and low back pain. The reason may lie behind the protective anteversion of the pelvis, which is caused by hip pain. Some authors assume that the flexion contracture of hip joint could contribute to compensatory flexion of lumbar vertebrae, and then cause the pelvis to tilt forward. Because of the reduced hip joint pain and improved hip joint function, the pelvis can return to a neutral position or slightly anteroposterior position after operation. Therefore, in our study, in addition to spinal deformity and stiffness, we also considered the influencing factor of the hip joint ,thus dividing the patients into five groups. Given that the flexion contracture of the hip less than 15° is often easily compensated by the lumbar spine , in this study, the patients with flexion contracture greater than 15° were included in the flexion contracture group. For the range of motion of lumbar, Dorr team regarded the change of sacral tilt angle above 35°as high range of motion, below 20°as stiffness, and 20° to 35° as normal. We divided the patients into five groups: group A: no flexion contracture of hip joint - good motive lumbar; group B: no flexion contracture of hip joint – limited motive lumbar (B1, APPt >0°; B2, APPt< 0°); group C: flexion contracture of hip joint - good motive lumbar; and group D: flexion contracture of hip joint -poor motive lumbar.
- Changes of sagittal parameters of spine pelvis in different follow-up times after THA.
The sagittal parameter of spine pelvis varies with follow-up time after THA. The follow-up of Taki et al.revealed that the pelvic sagittal plane tilt of patients after THA changed year by year, the change range of pelvic tilt was the largest one year after THA, and the plateau stage appeared one year later. However, some authors considered it was 3 months after operation[18, 19]. Similar results were found in our clinical observation. Additionally, this time was good for patients to comply with.
- Changes of pelvic sagittal rotation before and after operation in five groups
Group A: no flexion contracture of hip joint - good motive lumbar
Most of the patients developed osteonecrosis of the femoral head, DDH with osteoarthritis and hip osteoarthritis. The course of disease was relatively short and the patients were relatively young. The patients complained of hip pain, but the dynamic films showed, spine balance and no stiffness. Three months after operation, LL? and SS decreased, PT increased, and APPT gradually approached to 0°. For such patients, the changes of spine-pelvis sagittal parameters before operation resulted from hip pain, making the patients have protective pelvic anteversion, which is consistent with the literature report.
Group B: no flexion contracture of hip joint – limit motive lumbar (B1, APPt> 0 °; B2, APPt<0 °)
Most of the patients in this group were diagnosed with femoral head necrosis, DDH with osteoarthritis, primary hip osteoarthritis as well, often accompanied by low back pain, or lumbar surgery history. They were subdivided according to the direction of pelvic tilt. For patients with pelvic anteversion (B1), preoperative mild pelvic anteversion may be related to hip pain, that is, preoperative hip pain caused pelvic protective anteversion. However, due to the limited range of motion of the lumbar spine, the compensatory anteversion of the pelvis was also limited, and thus the pelvic anteversion of these patients was relatively small. With the gradual relief of hip pain after surgery, the anteversion of pelvis could gradually decrease. However, in our study, there was no significant difference between preoperative and postoperative parameters. We also found that about 15% of the patients had more than 10° after operation. The reason may be related to the degree of lumbar motion limitation. For this reason, we will further collect samples to explore the relationship between lumbar ROM and pelvic tilt. In patients with pelvic retroversion (B2), the kyphosis deformities, or the limitation of lumbar flexion and thoracic vertebra extension (flat back deformity) cannot compensate for the sagittal imbalance of the spine, causing the pelvis to be retroverted. Because only a small change was made to the pelvic retroversion even after surgery, sagittal imbalance of the spine was difficult to be corrected.
Group C: flexion contracture of hip joint - good motive lumbar
Hip flexion contracture can bring about abnormal sagittal parameters of spine- pelvis, which has been extensively recognized. However, it is also generally accepted that hip flexion contracture can lead to pelvic pronation, and some authors contend that patients with hip flexion contracture have pelvic supination changes.Our observation showed that patients with pelvic retroversion were usually accompanied by stiff spine, and the patients without stiff spine often had anteversion pelvis. The patients with larger flexion contracture (> 30 °) often contracted sagittal imbalance of spine, for the lumbar flexion cannot fully compensate for the hip flexion contracture. For such patients, together with the recovery of hip motion, the anteversion of the pelvis was significantly improved, PT was reduced, APPT was close to 0°, and SVA was close to normal. This is called pseudo- imbalance by some scholars.
Group D: flexion contracture of hip joint -pool motive lumbar
Hip arthroplasty can restore the hip rom, but cannot change the original deformity and stiffness of the spine, causing relatively poor outcome in patients, especially when ankylosing spondylitis involves the spine and hip. Such patients of ours had small SS, large PT, small LL, APP retroversion and SVA imbalance before operation. There was less improvement of pelvic retroversion after operation, and the spine was still in imbalance. The reason may be that the spine and pelvis were completely rigid.
- Functional outcomes in different groups
We also used the Harris score to investigate the functions of these patients before and after surgery.
For group A patients, the outcome after THA was good, and clinical results showed that Harris score was significantly improved. It might be because the matter of the patients is due to the hip, and the course of disease was short. After THA, the pain gradually disappeared and the function improved.
For group B1, in addition to the pathological changes of hip joint, there was limited lumbar ROM, and some patients underwent lumbar fusion, but the sagittal position of the spine was in a balanced state, so the patients' satisfaction was also higher. However, the Harris score was lower than that of group A, and the patients could not put on socks and shoelaces themselves, which may be related to the limited ROM of the lumbar and pelvis.
For group B2, the patients often had flat back deformity, and some patients were in an imbalance state in sagittal position of spine. They failed to restore their balance after THA, and needed to bend the knee to compensate. Therefore, the gait and function of the patients were poor.
Speaking of Group C patients with hip flexion contracture, although there was sagittal imbalance of the spine, no obvious deformity and stiffness of the spine were observed. The imbalance was due to the excessive flexion contracture of the hip, leading the lumbar flexion not to fully compensate, and the barycentre of the body to be still in front of the foot support area. This imbalance is usually called "false imbalance". After THA, the flexion contracture of the hip joint was relieved, and the spine was restored to balance. Harris score improved greatly and patients' satisfaction was higher, which is consistent with the literature reports.
Most of group D patients had ankylosing spondylitis involving the hip and spine. To maintain the balance of the body, the pelvis will be backward, and the knee joint will be flexed. Patients’ hip and spine are mostly stiff. Hip arthroplasty can improve the function of hip joint, but it cannot change the imbalance of spine. The improvement of Harris score was relatively small in our study.