1.Osseous implant capping
Acetabular implant capping can provide lateral support to the acetabulum and provide some bone volume for revision. The literature shows that it has good early and long-term results.
The acetabular cap is made from an autologous femoral head (allograft bone is rarely used) preserving the subchondral bone, which must be located at the superior edge of the acetabular defect area or within the acetabulum. A 3.2 mm drill is used to drill vertically and obliquely through the bone graft into the pelvis and fix it with 4.5 mm AO cancellous bone screws. A granular bone graft was placed in the gap between the bone graft and the host bone to improve bone healing (also known as a flying-buttress graft). All bone blocks healed with the host bone, so the authors recommended the use of a biologic socket cup and ensuring that the bone graft covered less than 50% of the prosthetic area. At an average follow-up of 14 years in this series, the bone block healed to the host bone in 93% of cases, and 10 hips were revised due to loosening of the acetabular prosthesis. In all cases, only two patients had structural bone grafting due to insufficient bone volume, indicating that the use of autologous structural bone grafting can provide reliable bone volume for revision, and the same results have been corroborated by other scholars.
2.Bone cement reinforcement and strengthening ring
The early efficacy of bone cement filling of supra-acetabular defects is encouraging. In two studies by Gill et al. using reinforcement rings or titanium cages, a total of 87 patients with severe DDH were reconstructed with Müller reinforcement rings (also known as roof rings) in the early stages, and more than 40 of them had granulated autogenous femoral head implants. Gill concluded that bone cement is not a substitute for bone grafting and can lead to aseptic loosening of the prosthesis. Subsequently, Gill fixed 33 hips (2 hips revised for aseptic loosening) with a Ganz reinforcement ring with a lower wing that hooked over the closure hole with good results.
Overall, it is significantly better to place the acetabular prosthesis at the level of the true socket and ensure a proper angle of inclination in DDH patients than to place the acetabular prosthesis in a high external superior prosthesis. When the acetabular prosthesis is not fully covered there are various options such as structural bone graft, cemented reinforcement or reinforcement ring.
2.4 Femoral reconstruction
In patients with severe DDH, the small femoral medullary cavity, abnormal development, excessive anterior tilt, posterior position of the greater trochanter, and possible osteotomy interfere with the implantation of the femoral prosthesis. Intraoperatively, if the center of rotation is shifted down, a femoral shortening osteotomy may be required to prevent vascular nerve injury, especially to the sciatic nerve, due to excessive distraction. A guide pin may be used to position the bone during reaming to avoid penetration of the femoral cortex.
A small standard femoral prosthesis may be used for mild DDH patients, while a straight stem prosthesis with a small and narrow medial curve may be used for severe DDH patients, as these patients tend to retain minimal femoral spacing after femoral neck osteotomy. Femoral rotational osteotomies are often required when the anterior femoral tilt is >40° and may require custom or modular femoral prostheses to adjust the anterior femoral tilt.
Currently, the use of a small straight tapered stem allows for rotation of the prosthesis within the medullary cavity to obtain the proper anteversion angle, thus allowing for less rotational osteotomy of the femur.
The importance of using a matched femoral prosthesis was first recognized by Silber and Engh, who reported that 16 of 19 patients required a matched prosthesis due to morphologic variation of the femur and flared changes in the enlarged metaphysis.
However, Charity et al. achieved good clinical results using a polished cement stem in combination with a subtrochanteric osteotomy.
Femoral osteotomies are often prone to sciatic nerve palsy when the center of the hip joint is at the level of the true socket, if the limb extension is greater than 100px in length. There are several ways to measure the length of the lower limb intraoperatively, but it has not yet been proven which method is superior. The required femoral shortening length is determined by the preoperative lower limb length and the intraoperative measured lower limb length.
Femoral shortening is often accomplished using either proximal femoral or subrotator osteotomies. Sub-rotor osteotomies may be horizontal, stepped, oblique, or double-wave osteotomies to correct angular or rotational deformities while shortening the femur, and some clinical reports support the use of these osteotomies.
In general, variation in proximal femoral morphology in patients with DDH often interferes with the placement of femoral prostheses; the use of modern histoplasty prostheses is beneficial; and adequate preoperative planning facilitates the determination of the femoral reconstruction plan and the possible need for femoral osteotomies.
3.Surface replacement
The third generation of surface hip prostheses uses cemented femoral prostheses and biologic acetabular prostheses, whose advantages include more retention of femoral neck length, lower friction, and reduced risk of dislocation due to the use of large diameter ball heads. These advantages make surface replacements more suitable for young DDH patients. However, early first- and second-generation hip surface replacement prostheses have been associated with higher rates of femoral neck fracture, metal allergy, and serum metal ion levels.
There are few reports of gold-to-gold surface hip replacements for patients with DDH who have developed inflammatory changes in the joint. Early reports showed that the use of gold-on-gold surface hip prostheses for mild DDH patients was prone to femoral neck fractures and femoral lateral prosthesis loosening, and Amstutz et al. found that the short-term results of using surface hip prostheses for DDH were similar to those of non-DDH patients, and that the acetabular lateral prosthesis had good fixation results over a follow-up period of 2-11 years. fixation was good. However, long-term results suggest that the use of surface replacements for DDH requires addressing stringent inclusion criteria, high long-term complications, and metal ion release.
Overall, the recent systematic review literature showed that no surface hip replacement prosthesis had a 10-year survival rate, and only 3 publications showed good 3-year survival rates for surface hip replacements. Therefore, the cost-benefit and safety of surface hip replacement needs to be evaluated.
Although the most common complication of surface replacement is aseptic loosening, the risk of femoral neck fracture remains high and the mechanism of occurrence is unknown. In addition, a meta-analysis that included 43 papers on surface hip replacement showed that although the functional outcome of surface hip replacement was better than or equal to that of total hip replacement, heterotopic ossification, aseptic loosening, and return rates were twice as high as those of total hip replacement.
4.Gold-to-gold joint replacement
The application of gold-to-gold surface hip prosthesis and total hip prosthesis has been controversial due to symptomatic inflammatory reactions around the prosthesis. Excessive acetabular tilt angles result in more marginal loading, which increases the wear rate and serum metal ion levels of the prosthesis and is associated with the development of periprosthetic inflammatory masses.
Recent removal studies have shown that gold-to-gold surface replacements have the same high wear rate as total hip replacements, with a higher incidence of collision-type marginal loading due to preservation of the femoral neck in surface replacements, especially when the cup placement is not angled enough. However, the acetabular prosthesis position alone did not affect the wear rate of gold-on-gold surface replacements.
Considering the safety and component benefit issues currently reported for gold-to-gold joints, gold-to-gold hip prosthesis replacement is not recommended for young DDH patients. In fact, in April 2010, the UK Medicines and Healthcare Products Regulatory Agency issued an issue of a medical device safety alert on gold-to-gold prostheses.
5. Clinical outcomes and complications
The complication rate of joint replacement in DDH patients is generally higher than in osteoarthritic patients, and this difference is not due to the younger age of DDH patients. The incidence of sciatic nerve palsy after joint replacement is approximately 10 times higher than in patients without DDH. Garvin believes that limb lengthening of less than 50 px is often safe, while Edwards believes that limb lengthening of more than 100 px greatly increases the risk of sciatic nerve palsy. Therefore, electromyographic testing and arousal testing can be performed in a manner similar to scoliosis orthopedics to prevent injury to the sciatic nerve. Intraoperative exposure and palpation of the sciatic nerve and maintaining knee flexion and extreme posterior extension of the hip are still required. If intraoperative neurasthenia is detected, a wake-up test is performed to determine if the sciatic nerve is injured. The arousal test is performed by lowering the level of anesthesia and instructing the patient to dorsiflex the ankle joint to check the functional status of the sciatic nerve. Preoperative communication with the patient is required to explain the need for this test. Also, the ipsilateral knee should be kept in flexion after limb extension to reduce the tension of the sciatic nerve.
Previous reports have shown a high rate of arthroplasty dislocation in patients with DDH, mainly due to greater trochanteric discontinuity (so-called greater trochanteric escape) and impingement of the femoral prosthesis on the anterior acetabular column during hip flexion and internal rotation. The risk of impingement is greatest in high and internally displaced acetabulae, but the risk of impingement can be reduced by increasing the femoral eccentric distance.
Patients with severe DDH are susceptible to intraoperative femoral fracture, so special care is required during medullary preparation and the use of a guide pin to position the medullary cavity to avoid penetration of the femoral cortex. If intraoperative penetration of the femoral cortex is detected, the prosthesis should be longer than twice the diameter of the femur in the area of penetration and fixed with a homogeneous cortical bone plate.
Obese young patients are increasingly being replaced with arthroplasty, but recent data confirm that morbid obesity does not affect postoperative outcomes and therefore refusal of arthroplasty in DDH patients with only a high body mass index is inappropriate. However, obesity has been reported to be a risk factor for periprosthetic infection.
Reports have shown that patients with DDH are more likely to develop infections than patients with osteoarthritic hip replacements. A combination of factors such as long operative time, extensive exposure, multiple soft tissue stripping, and often bone grafting are responsible for these results.
6. Summary
The anatomical variation due to DDH varies, and a comprehensive preoperative plan must be performed to determine the appropriate prosthesis, surgical approach, and bone defect reconstruction. Current long-term clinical results support the use of a gold-to-polyethylene friction interface. However, both patient and surgeon preferences and habits can affect the outcome of hip replacement in adult DDH patients.