The most important finding of this study was that the clinical measurements and radiological assessments of both two groups almost restore to normal and no one showed complications at the latest follow-up. Besides, for the patients, the pain has relieved and the gait has improved as well as no complaining about self-perceived LLD. So, both two approaches are effective to increase cup coverage by host bone and reconstruct acetabulum. According to the current literature [10], because of abnormal anatomy, secondary osteoarthritis of DDH occurs at a relatively young age (an average of 42 years old) and for the younger population, the long-term survival of implant was reported to lower than that of general ones due to more activity [31,32]. Besides, for DDH Crowe II/III, the biggest challenge lies in cup coverage and acetabular reconstruction. Also, the study [33] has demonstrated that the rate of failure of the acetabular component showed a positive correlation with the severity of DDH. So, the surgeons are more concerned about how to obtain adequate initial stability of the cup in order to realize long-term survival of the acetabular component and postpone potential revision surgery.
There is no consensus on the position of acetabular component. And placement of the acetabular implant can be located in true acetabulum or high hip centre. High hip centre is defined as the perpendicular distance from the femoral head center to the inter-teardrop line more than 35 mm [24]. Some scholars [11,13,17,19,34] agreed on the inserting the cup into true acetabulum due to superior biomechanics, better fixation and more bone mass. Placing the implant at anatomical centre of the hip enables optimal abductor muscle function and the bone mass for fixation of the component is larger than at more proximal level [12]. However, we must address problems such as limb lengthening [35], nerve palsy [35], less coverage of the cup [19]. Gratifyingly, subtrochanteric osteotomy [10], structural autograft [17] and medial protrusio technique [11] have provided novel and valid methods. Additionally, creation of high hip center was also proposed for primary acetabular reconstruction and was reported good long-term outcomes and showed no difference in polyethylene wear [36,37]. Nevertheless, there are many other problems to face. Firstly, high acetabular reconstrution often results in high, lateral and oversized cup placement leading to the problems like fixation, primary stability and restoration of normal hip biomechanics [10]. Secondly, at this high level, the bone stock is insufficient and shearing forces on the acetabular component may lead to early loosening. Meanwhile, longer lever arm for body weight can result in excessive load to the hip joint [19,38]. Thirdly, longer prosthetic neck length used to balance leg lengths possibly leads to neck-liner impingement [13]. At last, the patients performed primary THA owing to DDH usually are younger than hip osteoarthritis and most of them likely need revision surgery, which may be more difficult owing to limited bone stock [17]. Bicanic et al [39] reported that every millimetre of lateral displacement of the acetabular cup compared with ideal rotation center resulted in an increase of 0.7% in hip load and every millimetre of proximal displacement an increase of 0.1% in hip load. This accounts for high rate of failure if place the cup component in high hip centre. Additionally, Chen et al [14] and Stans et al [40] have demonstrated that using a high hip centre during acetabular reconstruction in DDH patients had higher failure rate. So, based on these reasons, we have chosed anatomic placement of the acetabular cup. And the results of this study was that LLD was approximately 5mm and no one complained about self-perceived LLD, if we reconstructed acetabulum at true location. Also, aseptic loosening was not found at the latest follow-up. These results demonstrated anatomic reconstruction of acetabulum was a superior choice once again.
Currently, literatures [18,41] have proved medial protrusio technique was an effective method to increase the rate of cup coverage without bulk femoral head autograft. Medial protrusio technique includes controlled medial wall fracture, medial wall osteotomy and medial wall penetration [11]. And the reasons why we chose controlled medial wall fracture as cotyloplasty are the simplification and safety compared with other two techniques. Besides, the current literatures [18,41] have demonstrated that the long-term survival of THA performing controlled fracture of medial wall showed no difference compared with that of general THA at 10 years. For medial wall osteotomy, it’s a more technically demanding process and the thickness of medial wall should be not less than 10 mm according to Zhang et al [13]. For medial wall penetration, the loss of bone stock is more than medial wall fracture. And it’s easy to ream excessively leading to the damage of the acetabular rim. We have provided illustration (Fig. 5) of the comparison of three medial protrusion techniques. And we hope better explanations can be realized for the differences. To our knowledge, there is no report more than 10 years about the medial wall osteotomy and medial wall penetration demonstrating the comparable outcomes with controlled medial wall fracture. So, based on these factors, we thought controlled medial wall fracture is a better one for increasing cup coverage in THA with DDH.
Sufficient initial stability of cementless cup is imperative for successful osseointegration and good long-term survival, but medial protrusion may decrease this stability. So, what rate of medial protrusion of the cup is the best? Dorr et al [26] recommended this rate should be less than 45% and Kim et al [27] suggested it should be within 50%-60%. Besides, according to Zha et al [11], a prospective 6- to 9-year follow-up of 43 consecutive patients using medial protrusio technique in cementless THA for DDH demonstrated that the rate of medial protrusion more than 60% had a high aseptic loosening rate in midterm. Also, excessive rate of medial protrusion may possibly result in cup component migration into the pelvis. And the rate of medial protrusion of our study was (48±4)% and no complications were observed at the final follow-up. The purpose of medial protrusion was to increase cup coverage by host bone and realize initial cup stability. So, we considered it was reasonable to realize enough cup coverage with the least rate of medial protrusion. According to the published studies and the results of our research, we suggested that the rate of medial protrusion should be less than 60%. If needed, supplementary screws can be used for initial cup stability. Additionally, The amount of protrusion was evaluated by preoperative templating on AP radiograph. Also, the surgeon would conduct assessment again intraoperatively after standard acetabular reaming. Preoperative planning would provide reference for intraoperative amount of medial protrusion. And the operators can judge the amount of medial protrusion according to intraoperative fluoroscopy compared with preoperative planning. The final amount of medial protrusion was determined on the preoperative planning and intraoperative evaluation.
For patients diagnosed with DDH, the abnormal femoral morphologies contribute to the complexity of the cases, which include excessive femoral anteversion, coxa valga and small diaphyseal diameters [42]. For these cases, modular stem like SROM stem (Depuy, Warsaw, IN) may be needed. Compared with monobloc stem, modular stem provides intraoperative flexibility in femoral reconstruction. Sleeves with various height and width options can be chosed to accommodate different metaphyseal morphologies. Also, the different combinations of neck and stem can optimize offsets, avoid LLD and match different medullary cavity [43.44]. If subtrochanteric shortening osteotomy was performed, modular stem like SROM stem can provide rotational stability [5]. In brief, modular stem possesses advantages including accommodating abnormal femoral medullary cavity, optimizing offset, avoiding LLD and providing rotational stability. However, there are concerns associated with modular stem including junction failure and corrosion due to the slippage and micromotion occurred at the stem-sleeve interface [40,45,46]. The study from Bobyn et al demonstrated that the wear particle was not significant enough to cause osteolysis and loosening [47]. Also, Seufert et al [46] and Kong et al [48] have reported modular stem can successfully accommodate the distorted anatomy of the proximal femur and achieved optimal stem version as well as excellent clinical outcomes. So, Modular stem can be recommended as an alternative choice to reconstruct femora in THA for patients diagnosed with DDH.
The technique, controlled fracture of medial wall, has several advantages to deal with unsatisfying cup coverage compared with structural autograft with bulk femoral head. Firstly, it can simplify the operation and does not ask for special surgical instruments. Meanwhile, this technique does not prolong the operation time and cause obviously additional damage to the patients. Secondly, the area of operated medial wall belong to bony union because of the application of autogenous mud-like cancellous graft and protection of the internal layer of the periosteum. And the integrity of acetabular rim does not be damaged. So, there is little influence on the primary stability and bone ingrowth of the cup component. Thirdly, medialization of the cup component can increase the rate of cup coverage and decrease wear due to the increase of the abductor lever arm and decreased loading of the hip joint. However, the primary concern of this technique is possibly excessive medial protrusio resulting in unsatisfied primary stability of the cup component or disastrous migration of the cup into pelvis when postoperative weight-bearing exercises. Besides, the range of medial fracture is not easy to control. And The future revision arthroplasty in the patients operated with controlled fracture of the medial wall is another concern, especially the higher fixation problem and bone deficiency. For the mentioned concerns, we suggest that firstly don’t chase for excessive rate of medial protrusio if the cup component can realize enough rate of cup coverage and initial stability. Secondly, the application of autogenous mud-like cancellous graft and protection of the internal layer of the periosteum play a vital role in bony union of medial wall. Postoperative function exercise should be rational and not too be ambitious. Thirdly, the man-made fracture of medial wall could realize bony union by application of autogenous mud-like cancellous graft and protection of the internal layer of the periosteum. So, no excessive bone loss was compared with primary THA. Additionally, with the development of material science, we believe many other effective methods can handle the fixation problem and bone deficiency easily.
The limitations of the current study were the retrospective research with small population and relatively short period of follow-up and we couldn’t acknowledge the long-term outcomes. Additionally, when we conducted radiological assessments, we just used 2-dimensional images to accomplish evaluations, which might compromise the robustness of the final results. But, It was the first study to compare the effectiveness of controlled fracture of medial wall with that of structural bone grafting to increase cup coverage and reconstruct acetabulum. And the study demonstrated that controlled fracture of medial wall could increase the rate of cup coverage without technically demanding, which can act as a selectable method to increase cup coverage and reconstruct acetabulum.