Developmental hip dysplasia includes a group of related disorders: hip instability, subluxation, dislocation, and acetabular dysplasia. Many patients behave normally in childhood and gradually develop subluxation as they grow older, at which point some patients are prone to develop complete subluxation if left untreated, often followed by traumatic hip arthritis (osteoarthritis of hip, OAH) in middle-aged and young adults, manifesting as pain and limpness after exertion and distance travel, with patients often starting to experience pain at around 30 years of age. Young, painful symptoms can still be tolerated.
How to treat this is more troublesome, and the long-term effect of surgery needs to be further observed and confirmed. The course of the disease progresses quickly, the pain is great, and serious people lose their ability to work. At this time, conservative treatment is often difficult to work, acetabular dysplasia secondary to hip osteoarthritis is a common disease, mostly due to untimely treatment at a young age, if the problem can be detected in time when young, early use of the pelvis, the proximal segment of the femur a variety of osteotomy, some patients can avoid or delay the occurrence of late osteoarthritis.
The diseased joint in patients with hip dysplasia can manifest itself in many forms of anatomical abnormalities. It ranges from a mild subluxation to a total dislocation with complete loss of contact between the femoral head and the true acetabulum. In general, the surgical procedure for total hip replacement in patients with hip subluxation is not very different from a normal initial total hip replacement and the operation is not very complicated. However, for complete dislocation of the hip joint and upward migration of the femoral head, the surgical difficulty is significantly increased, and the incidence of postoperative complications also rises.
1.Classification method
Most of the grading methods of hip dysplasia are based on the degree of dislocation of the femoral head relative to the acetabulum. The three commonly used grading methods are Hartofilakidis method, Eftekhar method and Crowe method. Among them, Crowe’s method is the most commonly used in clinical practice, which is a good reference for preoperative planning and determining surgical complications for the operative period.
Crowe type I: The degree of hip dislocation is less than 50%. The symptoms appear late, mostly between 50-60.
Crowe type II: the degree of dislocation is 50-75%, usually both lower limbs are equal in length and the bone reserve is good.
Crowe type III: 75%-100% dislocation. Patients with Crowe type II and III have an early onset of secondary arthritic symptoms, mostly between 30 and 40 years of age. Once arthritic symptoms appear, the disease progresses more rapidly.
Crowe type IV: safe dislocation. However, the hip may move well (pseudoarthrosis) and the local pain symptoms are not severe, especially in those patients who have lesions in both hips. Patients with acetabular tend to have early onset of clinical symptoms and poor prognosis.
2.Anatomical changes
Hip dysplasia causes a variety of local deformities that pose many difficulties for surgical repair. These structural abnormalities include shrinking and deforming of the femoral head and shortening of the femoral neck with different degrees of abduction and anteversion deformity. The greater trochanter is small and significantly posteriorly positioned. The medullary cavity of the femur is narrow, sometimes only about 1.5 cm in diameter 2 cm below the lesser trochanter. The proximal third of the femur is bent forward. The degree of anteversion of the femoral neck varies widely among individuals and can reach 90 degrees in severe cases, but there is no direct correlation between the magnitude of this angle and the severity of the disease.
The site where torsion occurs is primarily between the lesser trochanter and the isthmus of the femur. The true acetabulum is usually superficial, with a thin anterior wall, a thick posterior wall, and a narrow anterior-posterior diameter. The joint capsule is hypertrophic and elongated, and its lower portion may adhere to the pelvic wall, preventing the dislocated femoral head from entering the true acetabulum. The upper edge of the true acetabulum is sloped due to wear and tear of the femoral head, with bone defects and osteosclerosis. In cases where the femoral head is dislocated and displaced superiorly, there may also be a combination of functional shortening of the adductor muscle group and the N cord, iliopsoas, and quadriceps muscles, causing surgical inconvenience.
In cases of complete unilateral hip dislocation, there is usually a combination of lateral spinal recess and ipsilateral knee deformity. In addition, the upwardly displaced femoral head causes the hip abductors, which are normally oblique and inferior, to become horizontal and become the top block of the pseudo hip joint, which can easily lead to local misinjury if this anatomical variation is not well understood during surgery.
Along with the upward shift of the femoral head, there is a corresponding change in the position of the neurovascular surrounding the hip joint, especially the deep femoral artery and the femoral nerve. The femoral nerve may change from its normal downward lateral pathway and go superior lateral to the pelvis. The deep femoral artery may appear below the acetabular rim, which normally hates the passage of large vessels. In patients with severe acetabular dysplasia, these vascular-neural injuries are not uncommon and therefore require significant attention from the surgeon.
In addition to the anatomic variation caused by the disease itself, a history of previous local surgery also causes some alteration of the anatomy. This should be well understood by the surgeon.
The CE angle is an important radiographic indicator that defines the extent of acetabular dysplasia. The angle between the two lines is the CE angle, which is normally 25 degrees or more. If it is less than 20 degrees, it is severe acetabular dysplasia.
3. Indications: Artificial total hip arthroplasty should be considered only when there is severe joint pain and dysfunction and the patient is unable to complete basic daily life movements.
The surgeon must be clear about the complexity of arthroplasty in patients with acetabular dysplasia and the high incidence of postoperative complications, and strictly grasp the indications for surgery. Patients must be informed that hip arthroplasty for hip dysplasia is inherently risky, especially in those young patients who require a high level of postoperative activity. The mere correction of a limp, restoration of lower extremity isometric length, or simply to forego cane support are not sufficient reasons for total hip replacement.
In younger patients, various conservative treatments, including weight bearing reduction, anti-inflammatory medications and rest, and physical therapy, can be used in order to delay artificial hip replacement, and proximal femoral osteotomy around the acetabulum can also be effective in improving function and reducing symptoms.
It should be noted that most patients with bilateral hip dislocations, despite obvious body sway during walking, usually have few local joint pain symptoms and do not need crutches or cane support, and their joint function basically meets the needs of daily life. For these patients, simple gait abnormalities do not have indications for surgery.
4. Pre-operative preparation and prosthesis selection: including routine anterior-posterior, lateral and oblique pelvic views, plus CT imaging if necessary, to clarify the acetabular depth, size, bone condition, anterior tilt of the femur and femoral neck, width of the medullary cavity, and size of the femoral head neck, to guide contraction and help select prosthesis. Due to the large individual differences in the degree of hip pathology, adequate preoperative preparation of the prosthesis is required.
On the acetabular side, it is important to determine whether there is a bone defect in the true acetabulum and whether there is a sufficient bone bed to support the prosthesis. If the bone bed at the true acetabulum can only cover less than 30% of the surface area of the prosthesis, bone grafting, special acetabulum, or upward displacement of the acetabulum are required to compensate. To improve the bone bed coverage of the acetabular prosthesis, a small size acetabular prosthesis, such as 50 mm in diameter or smaller, is often used, so the corresponding preoperative preparation of a small 22 mm diameter head is required to ensure that the acetabular polyethylene lining is of sufficient thickness. Acetabular prostheses are usually fixed with non-cemented fixation.
There are three main problems on the femoral side, namely transitional anteversion, medullary stenosis and shortening of the femur. Usually the acetabular prosthesis is fixed with non-cemented fixation, while the femoral prosthesis can be based on the patient’s age, activity level, bone condition, and based on the operator’s personal preference. Our clinical practice is more likely to use uncemented prostheses, especially for patients who also require proximal femoral osteotomy. In order to meet the needs of the various situations encountered during surgery, it is advisable to prepare a combination prosthesis in addition to the conventional prosthesis.
In patients with unequal lower limbs, it is necessary to assess whether the discrepancy is related to extra-hip factors such as pelvic tilt and spinal deformity, in addition to the hip joint itself (e.g. femoral head dislocation upward), and to accurately calculate the actual degree of shortening of the lower limb. The anatomical characteristics of DDH are a shallow acetabulum with a thin anterior wall and a thick posterior wall, accompanied by a hyperplastic bone. In particular, in Crowe type III and IV patients, the acetabulum is large and develops in an oblique row; in type IV, the true acetabulum is small and sometimes cannot accommodate a new acetabulum; in addition, the femoral portion has a large anteversion angle.
For severe (type III and IV) patients undergoing acetabuloplasty, the entire joint capsule needs to be removed, the large and shallow pseudosockets and the smaller developing true socket need to be thoroughly exposed, the true transverse acetabular ligament is sought as the lower edge of the forming socket bed, the acetabular file is made at 45° to the patient, pointing 10° in the direction of the sacrum to deepen the socket bed, and because the anterior wall of the socket is thin and the posterior wall is thick, the force should be directed posteriorly so that the anterior wall retains as much of the bone bed. To prevent postoperative joint dislocation, the prosthesis should be placed in the position of the true socket as much as possible. Since the affected femur of patients with anterior hip is often in the externally rotated position, the direction of the femoral medullary file should always be aligned with the position of the medial femoral condyle when the femoral stem prosthesis is fixed to prevent dislocation caused by increasing the anterior tilt angle.
The smaller size of the acetabular prosthesis was used as much as possible, especially for type III and IV patients, because the true socket is small and has little bone in the bone bed, and using too large an acetabular file may result in the loss of surrounding bone, making it impossible to fix the acetabular prosthesis or increase the bone graft. Bolder et al. reported good clinical outcomes in patients without intraoperative implants when comparing patients with and without implants; in contrast, patients with large implants had unsatisfactory clinical outcomes.