dislocation after THR

A 73-year-old man with a history of hypertension and hyperlipidemia presents to your office with right hip pain that he feels is a functional impairment and with radiographs demonstrating severe osteoarthritis of the right hip. During your

discussion about total hip replacement, he states that he would like you to perform his surgery via an anterior approach because he has heard that it leads to a lower dislocation rate.

All of the following technical factors reduce the risk of dislocation in total hip arthroplasty except for:

1. Placing the acetabular component in 40 to 50 degrees of abduction

2. Use of a larger head size for a given neck diameter

3. Use of an implant with a skirted head

4. Use of an implant with a trapezoidal neck

5. Increasing the offset of the femoral component

 

Discussion

The correct answer is (C). The head-to-neck ratio of an implant plays a significant role in the risk of dislocation. The larger the head-to-neck ratio is for a given femoral component, the lower the risk for dislocation secondary to impingement. Increasing the size of the head for a given neck diameter (B) increases the head-to-neck ratio, whereas use of a skirted head (C) decreases the head-to-neck ratio. Correct component positioning (A) is fundamental to total hip arthroplasty and has been repeatedly demonstrated to play a crucial role in minimizing the risk of dislocation. Increasing the offset of the femoral component (E) lowers the risk of dislocation both by increasing the tension on the abductors and by decreasing the risk of impingement.

Which intervals are utilized to access the hip joint via an anterior approach?

1. Between sartorius and tensor fascia lata superficially/rectus femorus and gluteus medius deep

2. Between sartorius and rectus femorus superficially/tensor fascia lata and gluteus medius deep

3. Between gracilis and tensor fascia lata superficially/rectus femorus and gluteus medius deep

4. Between sartorius and tensor fascia lata superficially/rectus femorus and gluteus maximus deep

5. Between gluteus maximus and tensor fascia lata superficially/gluteus medius and gluteus minimus deep

Discussion

The correct answer is (A). The anterior (Smith–Peterson) approach to the hip utilizes the inter-nervous plane between sartorius (femoral nerve) and tensor fascia lata (superior gluteal nerve) superficially and rectus femorus (femoral nerve) and gluteus medius (superficial gluteal nerve) deep. The anterior approach to the hip is the only approach to the hip to utilize a true inter-nervous plane.

Which of the following choices correctly pairs the level of safety and the structures at risk with regard to acetabular screw placement during total hip arthroplasty?

1. Anterosuperior—safe—external iliac vessels

2. Anterosuperior—dangerous—obturator neurovascular bundle

3. Posterosuperior—safe—sciatic nerve and superior gluteal neurovascular bundle

4. Posterosuperior—safe—sciatic nerve and internal iliac vessels

5. Posteroinferior—safe—obturator neurovascular bundle

 

Discussion

The correct answer is (C). The acetabulum can be divided into quadrants by drawing a line from the anterior superior iliac spine through the center of the acetabulum and drawing a second line through the center of the acetabulum that runs perpendicular to the first. Radiographic and cadaveric studies have delineated the safety of, as well as the primary structures at risk of injury with, screw placement in each quadrant.

 

Quadrant	Level of Safety	Structures at Risk
Posterosuperior quadrant	Safest	Sciatic nerve
Superior gluteal neurovascular bundle Posteroinferior quadrant Inferior gluteal neurovascular bundle Anterosuperior quadrant	Safe   Dangerous	Sciatic nerve   External iliac artery and vein
Anteroinferior quadrant	Dangerous	Obturator neurovascular bundle

 

After undergoing total hip arthroplasty via a posterior approach, the patient would be at the greatest risk of dislocation if he were to perform which of the following activities?

1. Putting on shoes and socks

2. Taking a seat on a low sofa

3. Sleeping on his back

4. Descending stairs

5. Walking at a brisk pace

 

Discussion

The correct answer is (B). Posterior hip precautions are to avoid flexion at the hip beyond 90 degrees, internal rotation at the hip past neutral, and adduction at the hip past neutral. Therefore, the patient would be at the highest risk of dislocation if he were to take a seat on a low sofa, resulting in hip flexion beyond 90 degrees. The other activities listed do not place the hip joint outside the range of motion dictated by posterior precautions.

 

Objectives: Did you learn...?

 

The causes of hip dislocation after THR?

 

 

Internervous plane of the anterior approach to the hip? Specifics of posterior hip precautions?