When it comes to recurrent dislocation of the hip after total hip arthroplasty, surgeons will find it helpful to identify the primary cause of the patient’s instability and correct the problem at the time of revision surgery.
"An algorithmic approach for understanding what is the primary cause of instability is the best method for managing these patients," Craig J. Della Valle, MD, said. "I think once you identify what the problem is, it becomes a lot easier to address and fix the problem at the time of the revision."
Della Valle noted there are six types of hip instability, of which the most common - type I - is malposition of the acetabular component. He recommended using CT preoperatively instead of plain radiographs to more definitely identify component malposition.
"Although a shoot-through lateral radiograph shows the position of the socket fairly well, it can be seen more precisely on a CT scan," he said.
"If the cup is malpositioned, treatment is straightforward: Revise the cup and place it in the appropriate anteversion. At the time of revision, we also routinely use the largest femoral head size possible to further decrease the risk of dislocation. Finally, given our good early experience with dual mobility for revision surgery, we would consider the use of a dual mobility bearing, as well," Della Valle said.
Malposition of the femoral component, which is type II hip instability, is less common and more challenging to manage, according to Della Valle.
"Taking out a well-fixed femoral component, for most of us, is a bit more challenging than removing a well-fixed socket," he said. "Again, I think another good reason to get a CT scan preoperatively [is] so you can take a look at the femur and not be surprised intraoperatively."
Optimize instability cases
A constrained liner is traditionally used to treat abductor insufficiency (type III), but, dual mobility can be used as well. According to Della Valle, constrained liners can increase implant stresses at the bone-implant interface, decrease range of motion and may not use crosslinked polyethylene. Simply using a larger head is not enough to treat abductor deficiency.
"I would emphasize that if you are going to use a constrained liner or dual mobility construct, for that matter, you need to optimize everything else at the time of revision surgery," he said. "You cannot have an acetabular component that is retroverted, put in a constrained liner and think that is going to be a solution for you."
Type IV hip instability - bony or soft tissue impingement - is rare and harder to identify. It can be treated by removing the sources of impingement and increasing femoral head size, according to Della Valle.
"Type V instability we are seeing more frequently. That is a dislocation associated with late wear of the polyethylene liner," he said, noting it is treated with a modular head and liner exchange and once again increasing the femoral head size during the revision THA.
Constrained liner for instability
Patients with type VI instability are the most challenging in many ways as they do not have a clear etiology for their dislocation.
"We have generally treated them with revision to a constrained liner or dual mobility articulation," Della Valle said.
In his presentation, Della Valle noted a failure rate of about 15% among patients treated at his institution for hip instability; the highest rate of failure was among the patients with abductor deficiency. The success rate was 90% when patients with abductor deficiency were eliminated from the analysis.
"Given the high rate of failure in this group when they were treated with a constrained liner, our group has looked to dual mobility as a potential alternative in this challenging subset of patients," Della Valle said.
When Della Valle and his colleagues tried to look at factors associated with success or failure of revision THA for instability, revising the acetabular component was the only factor that portended a better outcome, he said.
"[B]asically what that teaches me is, if there is any question intraoperatively about the position of that socket, you should go ahead and revise it. I think that again speaks to the utility of getting a preoperative CT scan so you know exactly what that position is and it is not something you are trying to judge intraoperatively," he said. – by Casey Tingle