Etiology
Hip dislocation is commonly seen in trauma patients[18]. It had already been well understood and even classified since the reports from Thompson and Epstein[2]. Normally, when a traumatic hip dislocation was identified in a patient, emergency closed or open reduction would be performed to restore the normal anatomy of the hip joint, relieve the symptom of hip pain, and avoid the potential possibility of avascular necrosis of the femoral head[2, 19]. Even in some rare situations in which recurrent posttraumatic dislocation of the hip joint was identified, periacetabular osteotomy (or some other procedures) could be performed to stabilize the hip joint[12]. However, patients in this study were not under normal conditions. In this study, all patients had a history of high-energy trauma followed by hip pain and restricted movement. Some of them also had objective evidence to demonstrate hip dislocation even with an acetabular fracture. However, as China is still a developing country, due to certain economic or medical-service restrictions, these patients might not be able to receive proper initial medical treatment, resulting in a persistent condition of hip dislocation[6]. Alternatively, in some rare situations, it is suspected that the hip joint might have been initially reduced at the time of injury but that recurrent posttraumatic dislocation of the hip joint also occurred, causing a condition of persistent hip dislocation[12]. According to the radiological features that were identified before arthroplasty and characterized as femoral head dislocation and integral pseudoacetabulum formation, we suspect that in the period from the initial injury to the arthroplasty surgery, the hip joint was continuously in a dislocated condition[9–11]. After the onset of secondary osteoarthritis[20, 21], pain and restricted movement could be identified in these patients.
Radiological diagnosis
If there was a clear history that indicated that the patient had experienced a hip dislocation without a sufficient procedure to reduce the hip joint, the diagnosis might not be difficult with adequate radiological examinations including CT 3D reconstruction of the hip joint. However, in cases of low socioeconomic status, these patients often might not provide a traceable history clearly enough to enable surgeons or radiologists to make the correct diagnosis[6]. In addition, because the pseudoacetabulum forms a barrier at the top of the femoral head, the upward shift distance of the femoral head is commonly not very high. In normal anterior-posterior X-ray examinations, this long-term unreduced hip joint dislocation with pseudoarthrosis resembles “normal” osteoarthritis. In this circumstance, the correct diagnosis becomes crucial. Therefore, the diagnostic value of three different kinds of common radiological examinations were evaluated in this study. The results (Table 2) demonstrated that with 3D reconstruction, CT scans could almost distinguish all these patients from those with other hip disorders (including osteoarthritis, DDH, etc.). In addition, we also believe that, in some patients, 3D printing technology could further help surgeons make the correct diagnosis and identify the original acetabular position, which might be helpful for identifying the implant position of the acetabular component when creating the preoperative plan. Thus, if available, a 3D printing model of the pelvis is recommended for each patient’s preoperative plan. Furthermore, we found two radiological characteristics that might be helpful for screening these patients (Fig. 3). The first is the “aureole” sign, which could be identified on normal anterior-posterior view X-ray examination. It is known that osteoarthritis is characterized mainly by narrowing of the joint space width and by osteophyte formation[22]. In patients with long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis, the femoral head is dislocated to the posterior direction and is sheltered by the anterior bony structures of the hip joint[6]. Meanwhile, the newly formed osteophyte (pseudoacetabulum) surrounds the femoral head. Therefore, in addition to the typical features of osteoarthritis, a semicircular high-density area could be identified around the femoral head in typical cases, named the “aureole” sign. The other radiological sign could be identified on axial CT scans. At the level of the rotation center, the anterior and posterior column and the internal wall of the acetabulum resembles a large letter “I” (in print) in normal people (or patients with osteoarthritis). However, in these patients, due to the upward shift of the rotation center and posterior osteophyte formation, the bony structure looks like a rhombus (at the level of the rotation center). This feature is called the “rhombus” sign. The posterior corner of the rhombus is the newly formed osteophyte (posterior wall of the pseudoacetabulum). We believe that understanding these two special radiological signs might be helpful for surgeons or radiologists to distinguish these patients from “normal” osteoarthritis patients. Note that these two radiological signs are subjective, abstract concepts, and there is no objective quantitative index to describe them. Therefore, we do not include these features in our diagnostic test.
Radiological measurements
In terms of preoperative radiological evaluation, moderate to integral formation of pseudoacetabulum (Fig. 2c) could be identified in all patients. The time when the pseudoacetabulum was formed remained unknown consequent to the deficiency of the patients’ preoperative longitudinal data. In reports from Nagi[14] and Ilyas[17], which characterized the time period from initial injury to surgery as less than one year, no pseudoacetabulum was mentioned. In contrast, according to the report from Pai[6], in four patients with unreduced hip dislocation more than one year, a pseudoacetabulum was formed. In our study, with minimally three years from initial injury to surgery, all patients had significant pseudoacetabulum formation. Therefore, we hypothesize that the time needed for pseudoacetabulum formation was approximately one to three years. The other important issue that we found in the preoperative evaluation was atrophy of the original acetabulum (Fig. 2d). In some patients, the diameter of the original acetabulum was significantly lower than that of the contralateral acetabulum. The reason is not clear, and we have not found related reports. We hypothesize that this phenomenon might be associated with a lack of stress stimulation from noncontact between the original acetabulum and femoral head. Or, in some cases combined with acetabular fracture, the “atrophy” is the outward appearance of bone deficiency of the original acetabulum. These changes might have an influence on the patient classification.
Classification
To help surgeons and radiologists understand the real pathoanatomical nature of patients with long-term unreduced hip joint dislocation, secondary osteoarthritis and pseudoarthrosis, as well as to make recommendations for surgeons to address this type of patient, we classified the patients into three types (four subtypes). Since in these patients, the femoral head is in a position of posterior-superior dislocation relative to the original acetabulum, a pseudoacetabulum is formed on the posterior-superior direction of the original acetabulum[6]. The patient's classification and treatment are then determined by the relative positional relationship between the pseudoacetabulum and the original acetabulum. The original acetabular atrophy also influences the classification of patients. In the first case (type A), the original acetabulum retains its original size. Because the distance of the posterior dislocation of the femoral head is relatively far from the original acetabulum and the diameter of the pseudoacetabulum is relatively small, there is no pathological anatomical relationship (contact) between the pseudoacetabulum and the original acetabulum. Therefore, there is no atrophy and no bone deficiency of the original acetabulum (which means the original acetabulum is nearly normal). In the second case (type B1 or B2), the dislocation distance between the femoral head and the original acetabulum is closer, or the diameter of the pseudoacetabulum is larger. At the same time, when the diameter of the original acetabulum maintains its original size, as a result, the anterior-inferior wall (osteophytes) of the pseudoacetabulum invades the posterior-superior part of the original acetabulum. After reaming of the original acetabulum, there is a bone defect in the posterior-superior part of the bone socket (type B1). In the other case, although the anterior-inferior wall of the pseudoacetabulum invades forward, the original acetabulum is atrophied, which helps it “escape” the invasion of the pseudoacetabulum. In this situation, there is commonly no significant bone deficiency on the posterior wall or dome area of the original acetabulum, but the original acetabulum is usually in a small diameter state (type B2). In the last case, the entire original acetabulum is nearly full-filled with the anterior wall of the pseudoacetabulum when the pseudoacetabulum continues to enlarge and to form new osteophytes. The rotation center position of the newly formed hip joint (pseudoarthrosis) will be located at the position of the posterior wall of the original acetabulum. In this situation, after reaming of the original acetabulum, there would be a large bone deficiency in the bone socket of the original acetabulum; or the original acetabulum could not be identified during surgery (type C).
Surgical treatment
The main purpose of classification is to guide the performance of total hip arthroplasty in these patients (Table 6). For such patients, the treatment methods could be generally divided into two types: hip joint reduction and artificial joint replacement. Since there was a long time (at least 3 years) between the initial injury and the surgery, as well as findings that indicated the onset of hip osteoarthritis, we did not attempt reduction surgeries in these patients[6, 13]. All patients had undergone total hip arthroplasty. There are some uniform characteristics which should be noted during surgical process of total hip arthroplasty in these patients as following: (1) the “pseudo” acetabulum might be mistaken for the original acetabulum, leading to a malposition of acetabular component; (2) the original acetabulum might be atrophied and covered under soft-tissue (similar to the situation in DDH patients), making it difficult for surgeons to identify the original acetabulum; (3) bone deficiency might be identified; (4) the process of reduction might be laborious.
For patients with type A injuries, “standard” surgical procedures could be performed since the original acetabulum remains nearly normal. For patients with type B1 injuries, we expected to identify a bone deficiency in the posterior-superior part of the bone socket after reaming the original acetabulum. In this situation, the “standard” acetabular component could be placed on the original acetabular bone socket with structural bone grafting or augmented to fill the bone deficiency area[23]. Similarly to our study, Ilyas[17] has reported that the femoral head might be an optimum selection in this situation. In that study, 13/15 patients with old, short-term hip dislocations (time from injury to surgery equaling less than 1 year) received total hip arthroplasty with the femoral head as a structural bone graft. For patients with type B2 injuries, because the acetabular rim remains integral, an acetabular component with a small diameter (often ≤ 42 mm) could be placed on the bone socket of the original acetabulum without structural bone grafting. Generally, surgical procedures for implanting the acetabular component for patients with type B injuries are similar to those for patients with developmental dysplasia of the hip[24]. Patients with type C injuries are characterized by severe original acetabular bone deficiency or even a failure to identify the original acetabulum during surgery. We only included one patient with type C injuries in this study. In this case, the acetabular component was placed on the original acetabular position, with multiple augmentations to help fill the severe bone deficiency[25]. Despite the lack of objective evidence, we believe that if the original acetabulum cannot be identified during surgery, the acetabular component could be placed on the bone socket of the pseudoacetabulum[26], while a restrictive liner could be used to avoid potential instability and dislocation of the hip prosthesis resulting from the retroversion of the acetabular component. For patients with type C injury, the surgical procedures for implanting the acetabular component are similar to revision surgery[27] (Fig. 6).
There are another two points to which surgeons should pay additional attention. The first is the selection of a surgical approach. As the direct anterior approach has become an increasingly popular, minimally invasive technique because of decreased pain and accelerated functional recovery, it has become more commonly selected by surgeons for performing total hip arthroplasty in recent years[28]. However, even though three patients underwent surgery via the anterior approach in our study, we do not recommend this technique for these special cases. In such patients, as a consequence of posterior dislocation of the femoral head as well as the blocking of anterior osteophytes of the pseudoacetabulum, the femoral head might be difficult to remove following femoral neck osteotomy when the total hip arthroplasty is performed via the anterior approach. In addition, the walls (osteophytes) of the pseudoacetabulum cannot be well exposed and removed via the anterior approach. Therefore, a posterior approach is recommended for performing total hip arthroplasty in this type of patient. The second point to note is that we recommend that the pseudoacetabulum (osteophytes of secondary osteoarthritis) be completely removed from the surface of the ilium. This is crucial before the process of reducing the prosthetic joint. In fact, although the dislocation distance between the femoral head and original acetabulum is not very far, it is commonly very difficult when surgeons are trying to reduce the hip joint in these patients during surgery. Since the gluteus medius and gluteus minimus are located on the surface of the ilium, when the osteophytes of the pseudoacetabulum are formed, the gluteus medius and gluteus minimus will be pushed away from the surface of the ilium (Fig. 5). This causes the gluteus medius and gluteus minimus to “bypass” the osteophytes, prolonging the distance from the attachment point of the muscle (posterior-superior border of the ilium) to the tip of the trochanter, increasing the muscle tension and causing a failure of reduction. In Ilyas’s study[17], rather than the limited soft-tissue release in our study, they had performed extensive soft-tissue release including the capsule, iliopsoas, iliotibial band, rectus femoris and even hamstrings with another incision. In contrast, in our study, we found that after removing the osteophytes of the pseudoacetabulum, the muscle tension of the gluteus medius and gluteus minimus would be expected to decrease dramatically, making it easy to reduce the joint without over-release of soft-tissue or proximal femoral osteotomy (seen in the video).
Limitations
There are several limitations of our study, and we have enumerated some major ones. First, the sample size was small. In particular, when the sensitivity, specificity and accuracy were calculated, the insufficient sample size of patients might have resulted in misleading estimates of these indexes (especially the specificity). Second, the follow-up time was relatively short in this study, making us unable to determine the long-term postoperative prognosis and the survival rate of the prostheses. Third, because of the retrospective design of this study, there was a lack of preoperative longitudinal data from the patients. Therefore, we could not identify whether the femoral head was dislocated at the time of initial injury (primary dislocation) or gradually after the injury (secondary dislocation) in some of our patients; furthermore, we could not provide objective evidence to demonstrate how the pseudoarthrosis formed, why there was acetabular atrophy in some patients or what was the relationship between the pseudoarthrosis and the onset of hip pain in the patients. Additionally, when a hip dislocation was identified without emergency reduction, the incidence of avascular necrosis of the femoral head might have been very high. However, our finding could not explain why no significant evidence of avascular necrosis was found in these patients.