CROWE Ⅳ DDH patients always displayed severe anatomical deformities on acetabular and femoral sides, including both bone and soft tissue deformities. Today, it is a generally held consensus that the hip rotational center should be reconstructed at its anatomical position for CROWE Ⅳ DDH patients. However, highly dislocated femoral head and severe perijoint soft tissue contracture render hip reduction a difficult process. Due to this, several osteotomy methods were recommended to aid in the hip reduction process [16–20]. With certain osteotomy techniques, hip reduction of CROWE Ⅳ DDH hips could be considered more trivial. However, with excessive osteotomy, achieving equal leg length post surgery for CROWE Ⅳ DDH remains difficult. Several non-osteotomy THA techniques were introduced, such as setting femoral osteotomy lines as low as possible, using powerful muscle relaxation medications [13], or using a Hohman retractor to help reduction [21]. Compared to THA combined with osteotomy, non-osteotomy techniques are widely considered to not sacrifice leg length. However, some research found that, except for highly dislocated femoral head, bony leg length discrepancy [9] and malformation of the pelvis[8] also affect post surgery leg length of CROWE Ⅳ DDH patients. In this study, for patients in group A, the tibial and full leg length of the affected side was significantly longer than that of healthy side. Furthermore, the distance from the teardrop to the line connected to most inferior points of the sacroilium joint of the affected side is also significantly larger than that of healthy side. Compared to a healthy hip, after anatomical hip rotational center reconstruction, a more inferior anatomical acetabular position combined with a longer leg in bony length could easily lead to an obvious longer functional leg length of the operated side after THA. In this condition, osteotomy should considered, not only to aid in hip reduction, but also to help balance functional leg length.
For CROWE Ⅳ DDH patients, there could be many LLD definitions, such as bony LLD, functional LLD and anatomical LLD[7]. Therefore, choosing optimal LLD to balance is a careful process. Y Li et al [7] described a lower limb balance strategy for CROWE Ⅳ DDH patients in detail. According to unilateral side or bilateral side DDH, they divided patients into type Ⅰ(unilateral) or typeⅡ (bilateral), and patients are further subdivided into 3 subtypes according to pelvic oblique and fixed spinal curvature. If there were no bony LLD, this was noted for patients without pelvic oblique to balance functional LLD, for patients with compensatory pelvic oblique, they noted to balance anatomical LLD is optimal. In that previous research, there was an overall significant bony LLD of 3.5mm, however, they concluded that it had little clinical significance, besides they did not take pelvic malformation into consideration. At the onset of this study, we excluded patients with lumbar spine stiffness (lumbar spine lateral curve <15°) or severe spinal deformity. After taking bony LLD and pelvic malformation into consideration, we reason that functional LLD is the most appropriate LLD to balance, and we agree that with bilateral CROWE Ⅳ DDH patients, the same femoral procedure is important. There are several reasons for choosing functional LLD to balance in our study. Frist, after functional LLD balance, patients could have a high degree of satisfaction of leg length after surgery at once, because functional LLD is the length they wanted to lengthen with the affected leg. Second, after taking bony LLD and pelvic malformation into consideration, and completed balance of functional LLD, the line connected to most inferior points of the sacroilium joint would be horizontal, which generally lead to most appropriate positions of lumbar vertebra. And if there are no bony LLD and pelvic malformations or these factors have a little influence, the functional LLD is almost equal to femoral head dislocated height, and this opinion is verified by Y Li[7]. Third, with bony LLD and pelvic malformation, choosing femoral head dislocation height or bony LLD to balance is not only inaccurate and could lead to functional LLD post surgery, it would also take a longer time for patients to adapt.
However, in this study, not all of the patients could gain equal leg length at the final time of leg length comparison after reduction due to excessive tension of soft tissue. At the last follow up, functional LLDs for the 3 patients were 0.3cm, 0.5cm, and 1.0cm. For the patients with 0.3cm and 0.5cm functional LLD, no other treatment measures were needed; for the patient with a 1.0cm functional LLD, shoe lift was recommended to improve limb function.
Other than femoral head dislocation height, soft tissue contracture, bony LLD and pelvic malformation, the hip rotational center can also affect leg length. In cases with small and shallow acetabular, it is difficult to predict cup size and position of CROWE Ⅳ DDH patients during preoperative planning process [22]. After surgery, we measured vertical distance from cup center to teardrop line for every hip in two groups, and found that the height of hip rotational center of group A was significantly smaller than that of group B. This result indicates that a more inferior hip rotational center could lead to a longer functional leg length. In our opinion, a more inferior hip rotational center is related to a severely small and shallow acetabular of CROWE Ⅳ DDH patients, and due to insufficient bone stock of anterior acetabular column, the acetabular cannot be fully reamed in order to reserve the bone of the anterior acetabular column as much as possible.
Although some research notes that osteotomies should be chosen when leg lengthening is more than 3.0–4.0mm[25], we opted to choose osteomy according to operational conditions. The primary reason for choosing osteotomies is to protect against excessive stretching of the sciatic nerve. Although we agree that excessive stretching can be a primary factor in sciatic complications, the threshold of excessive stretching for high sciatic nerve complications remains unknown. On the other hand, in some studies lacking osteotomy, no permanent sciatic complication occurred [13,21]. In this study, only 7 of 17 CROWE Ⅳ DDH received subtrochanter osteotomy, and the overall mean osteotomy value was 1.16 (range0–3.7cm).
There are several limitations to the scope of this study. First, the sample size is relatively small (17 hips for group A and 20 hips for group B). However, CROWE Ⅳ/ⅠDDH patients combined with other CROWE type DDH hips were exclude, and the overall time we recruited patients was one full year. Second, we did not test intraobserver and the interobserver agreement during radiological measurements. However, these measurement methods are nonetheless in line with previous studies, and they have been verified as having strong reproducibility. Third, we did not gain full leg length radiographs post surgery, therefore we could not study any effect of stem position to leg length further.
In a sense, after elaborate preoperative planning and leg length comparison during the operation process repeatedly, combined with femoral shortening osteotomy if necessary, THA could balance functional LLD of CROWE Ⅳ DDH patients.