Prevention of Prosthetic Dislocation after Hip Arthroplasty
Hip replacement is one of the achievements of orthopedics in the last century and is the most successful and widely used procedure in orthopedics today. However, as one of the common complications after hip arthroplasty, dislocation of the prosthesis, when it occurs, can cause a great blow to the self-confidence of patients and doctors and affect the quality of life of patients. Early studies showed that the incidence of prosthesis dislocation after initial total hip arthroplasty ranged from 2% to 5%, and the rate of dislocation in revision surgery could be as high as 27%, with abnormal prosthesis position and low soft tissue tension being the main causes of dislocation [1-3]. With increased awareness, the incidence of postoperative dislocation has been mostly below 1% through detailed preoperative, intraoperative and postoperative management [4-6]. In this paper, measures to prevent postoperative prosthetic dislocation are reviewed.
1. Mechanism of prosthetic dislocation
Mechanically, dislocation is a movement of the center of the femoral head first vertically and then parallel to the acetabular surface, and is the result of two forces that promote and prevent dislocation against each other. The femur moves to a certain extent then the trochanter or femoral prosthesis impinges on the acetabulum and its surrounding soft tissues, and if the forces preventing dislocation fail to prevent this tendency, it can lead to dislocation of the prosthesis through leverage. Posterior dislocation is the most common type and occurs when the hip joint is flexed, pronated, or internally rotated beyond a certain range. It is followed by anterior dislocation, which occurs when the hip is posteriorly extended and externally rotated [7].
The stability of the joint is related to the accommodative relationship of the joint and the tension of the surrounding soft tissues. When the prosthesis is abnormally positioned, the joint has a poor accommodative relationship and the activities required for daily life can cause impingement of the femur or femoral prosthesis on the acetabulum, resulting in dislocation. Soft tissue tension is a factor against dislocation and the stability of the joint is poor at lower levels [8]. Prevention of postoperative prosthetic dislocation is achieved precisely by choosing the right type of prosthesis, improving the accuracy of prosthesis placement, increasing soft tissue tension and paying attention to the range of motion after surgery.
2. Pre-operative measures
2.1 Soft tissue tension judgment: patients with a history of previous hip surgery, patients with neuromuscular disorders (e.g. patients with cerebral thrombosis sequelae, patients with pediatric paralysis, etc.) and elderly people, etc., have lower soft tissue tension in their hips and have a higher chance of postoperative dislocation [6]. These patients should be examined in detail to understand the abductor muscle strength, and simple femoral head replacement and large head prosthesis replacement can be selected according to the situation. If the abduction strength is significantly reduced, a restrictive prosthesis or an intraoperative greater trochanteric advancement may be used to increase soft tissue tension [6, 9-11].
2, 2 Diagnosis of comorbid disorders: studies have shown that patients with comorbid lumbar disorders (e.g., ankylosing spondylitis), contralateral hip deformities, and knee disorders have an increased chance of prosthesis dislocation after total hip arthroplasty [8, 12]. These disorders may affect the intraoperative placement of the prosthesis and may also affect the postoperative pelvic tilt. Diagnosing these disorders and making the appropriate intraoperative adjustments can be of great significance in preventing dislocation. In patients with greater lumbar lordosis, the abduction angle of the acetabular prosthesis should be smaller and the anterior tilt angle should be slightly larger; in lumbar lordosis, the anterior tilt angle of the acetabulum should be smaller [8]. Disorders of the contralateral hip can also increase the incidence of postoperative dislocation. When the contralateral hip is tonic in the abductor position, the abduction angle of the acetabulum should be smaller; when it is tonic in the abductor flexor position, the anterior tilt angle of the acetabulum should be smaller; when it is tonic in the adductor position, the abduction angle of the acetabulum should be larger; when it is tonic in the adductor flexor position, the abduction angle of the acetabulum should be larger [13].
2, 3 Choice of prosthesis: for patients with low soft tissue tone, simple femoral head replacement and large head prosthesis replacement are selected. Patients with significant hip lesions, such as Crowe type IV developmental hip dislocation, require special types of prosthesis (e.g., grouped S-ROM prosthesis, Wagner prosthesis) in order to adjust the angle and neck length intraoperatively according to the situation [14] and reduce the incidence of dislocation. For patients with higher requirements for hip range of motion, such as young patients with femoral head necrosis, large head replacement or hip surface replacement can be used [11].
3, Intraoperative measures
3, 1 Patient position: oblique lateral position, because of the poor fixation, will lead to the change of patient’s position with the pulling of the surgical operation, which will affect the operator’s judgment and the placement of the prosthesis (especially the acetabular prosthesis) position. During lateral recumbency surgery, firm fixation can improve the accuracy of acetabular prosthesis placement [15], which can be fixed at the patient’s buttocks and pubic symphysis and posterior thorax, respectively, with little intraoperative postural change and high accuracy of prosthesis placement. The lateral position requires the trunk to be perpendicular to the ground and the longitudinal axis to be horizontal to the ground. For patients with unequal width of both shoulders and hips, a pillow can be placed under the narrow part. In the flat position, for patients with bilateral hip asymmetry, pillows can be placed under the atrophic side to maintain the pelvic level.
3,2 Surgical access: a review by Masonis et al [16] found that postoperative dislocation was related to surgical access, with a dislocation rate of 3.23% for the posterior lateral approach, 2.18% for the anterolateral approach, and 0.55% for the direct lateral approach. However, the posterior lateral approach is widely used because it does not damage the gluteus medius, and the dislocation rate of the posterior lateral approach does not differ from that of the other approaches when the posterior capsular suture is used [5].
3,3 Prosthesis placement: abnormal prosthesis position, especially acetabular prosthesis position, is the main cause of dislocation. Many scholars have been working on research to improve the accuracy of prosthesis placement. For total hip replacement without obvious lesions in the acetabulum, the prosthesis can be placed according to its normal contour. However, for patients who cannot be placed in an ideal position for various reasons, such as patients with lumbar spine deformity fusion or hip deformity fusion; patients with serious acetabular lesions, such as massive bone growth, acetabular dysplasia deformity, revision and hip fusion, the correct placement of the prosthesis is more demanding for the surgeon. Intraoperative marking and intraoperative fluoroscopy are more common and effective methods [17]. Computer-assisted navigation can also improve the accuracy of prosthesis placement [8, 18], but this method is more expensive and relatively complicated.
3, 4 Intraoperative determination and treatment of prosthesis stability
Widmer et al [7] used three-dimensional computer simulation to find that the acetabulum could provide the theoretical maximum range of motion for the hip joint from the perspective of the prosthesis design at 40° to 45° of abduction and 20° to 28° of anteversion. However, because of the discrepancy between the intraoperative seen prosthesis position and the postoperative position, the patient’s postoperative sitting, lying, and other daily activities can affect the pelvic tilt, the orientation of the acetabular prosthesis, and the safe range of motion of the prosthesis (see Figure 1) [18]. Impingement can occur not only between the femoral prosthesis and the acetabular prosthesis, but also between the femur (residual femoral neck, lesser trochanter) and the acetabulum and surrounding residual bone, spilled bone cement, and soft tissues. Computerized navigation simulation cannot remove the effects of soft tissue impingement and unrevealed bone fragments.
Intraoperative stability tests [19], in which the maximum range of motion of the hip joint in flexion, posterior extension, adduction, abduction, internal rotation and external rotation is checked with trial pieces to identify factors affecting hip stability and adjust accordingly, can improve the stability of the joint. The use of lengthened femoral head prosthesis, the placement of an anti-dislocation acetabular liner at the site of dislocation tendency, and the use of a larger diameter femoral head prosthesis (currently, the commonly used femoral head is 28 mm in diameter, which can be replaced by a 32 mm diameter femoral head) can effectively treat postoperative dislocation of the prosthesis and improve the stability of the hip joint. The lengthened femoral head prosthesis increases the Offset value, increases soft tissue tension, and reduces the chance of impingement between the residual femoral neck or lesser trochanter and the acetabulum, spilled bone cement, and soft tissue [7, 10]. However, an excessively long femoral neck prosthesis increases the force arm and increases wear on the acetabular prosthesis. A dislocated acetabular liner can limit the range of motion of the joint, and a misplaced position can instead increase the likelihood of dislocation (see Figure 2) [20]. A larger diameter femoral head prosthesis also does not increase joint stability when the abduction angle of the acetabulum is too large, or when soft tissue tension is very poor [21, 22].
Figure 1 Bilateral total hip replacement due to ankylosing spondylitis
Fig. 2c Intraoperative misplacement of the anti-dislocation liner was seen, and the acetabular liner was adjusted to increase the length of the femoral neck Radiographs 3 years after surgery, no further dislocation occurred 3.5
Soft tissue repair: in 395 patients without posterior joint capsule repair, Pellici et al [23] had 4% dislocation of the prosthesis; whereas in 395 patients with posterior joint capsule repair, no dislocation occurred. Showing that posterior joint capsule repair increases joint stability and reduces the incidence of dislocation, Morre et al [10] suggested the use of anterior transposition of the greater trochanter to improve joint stability in patients with poor soft tissue tone. However, this method has the potential for nonhealing and displacement of the greater trochanter.
4. Postoperative management
After surgery, prosthesis dislocation can occur when the patient is moved roughly because anesthesia has not fully recovered and muscle tone is low. Within 6 weeks after surgery, the incidence of dislocation is high, accounting for 70% of total dislocations, because the joint capsule has not healed and the granulation tissue has not formed a scar. Guidance on the range of motion for patients at this stage should be limited to light daily activities, with flexion not exceeding 90° and no cross-legged, excessive posterior extension, etc.
Zhao Fengzhao et al [19] found that: the stability test can simulate the condition during dislocation, and limiting activities in that direction can reduce the occurrence of dislocation. miki et al [24] simulated the activities after hip arthroplasty through computer navigation, and were able to find the dangerous range of activities that lead to dislocation, and concluded that it can be used to guide the range of activities of patients after total hip arthroplasty.
In conclusion, a detailed preoperative examination of the patient to understand the patient’s comorbidities and joint tension and to select the appropriate type of prosthesis; intraoperative improvement of the accuracy of prosthesis placement and appropriate adjustments according to stability; and postoperative guidance of the patient on the range of motion can reduce the incidence of prosthesis dislocation after total hip arthroplasty.