CONSERVE® Total Hip Arthroplasty Utilizing BFH® Technology
Harlan C. Amstutz, M.D.

Dislocation is a common complication after total hip arthroplasty (THA) and is the number two cause for revision. The reported incidence of dislocation after THA varies but is much more common than we thought. The incidence of dislocation in 58,521 THA in the Medicare population was 3.9% in the first 26 weeks after primary operation and 14.4% after revision surgery ⁹.

The rate of dislocation in 4,164 THA (all ages) performed at the University of Iowa was 7.2% after primary THA and 11.2% after revision THA ⁵. In 19,680 THA's performed at the Mayo Clinic (all ages) the dislocation incidence was 1.8% at one year but increased 1% for each five years reaching 7% at 25 years ¹⁰. The risk of dislocation with Osteonecrosis as a primary diagnosis was 14.6% compared to 6.4% for osteoarthritis. The occurrences of late dislocation occurred more often in younger patients and in women. The ball sizes in these studies ranged from 22-32mm.

The stability of a larger-diameter femoral head for THA has been recognized since the early ‘70s when we performed studies at UCLA ^2,3. A larger femoral head must travel a greater distance before subluxating or dislocating.  This enables a greater range of motion of the hip before the femoral neck (of the component) impinges on the acetabular component and levers the head from the shell. In our previous study, the prevalence of dislocation after 850 surface arthroplasties was 0.3% with head sizes ranging from 38 to 51mm (average, 46mm) and tends to support the use of the big or jumbo femoral heads to treat instability ¹.

In 1987, we first applied the concept of a big femoral head for implants in patients who had recurrent instability or chronic dislocation following THA. This “Tripolar” treatment consisted of a large inside diameter acetabular cup and a bipolar femoral head sized to approximate the diameter of the normal hip. The first publication describing this treatment in the very difficult revision situation was published in 1994 ⁶. This data has been updated recently with longer-term follow-up and was published in 2002 (Jumbo Femoral Head for Treatment of Recurrent Dislocation following Total Hip Replacement) ⁴.

Despite a number of improvements in femoral stem neck geometry and increasing femoral head sizes up to 36mm, dislocation continues to be a significant problem after THA.

New technology now facilitates very precise metal on metal bearings with extremely low wear, and there is evidence that larger ball sizes with a metal on metal bearing combination produce less wear than smaller balls. This is the opposite result of what we found with metal and polyethylene (plastic) combinations ⁷. Therefore, we have begun to apply the concept of a big femoral head for primary cases in an effort to eliminate dislocation. The indications for this type of surgery include all patients, but especially those of high risk (women, diagnostic categories of osteonecrosis, developmental dysplasia, neuromuscular imbalance or advanced age), and those who have had prior surgery.

By using the thin walled acetabular component from the CONSERVE^® Plus Surface Arthroplasty System for THA, the ball size may be increased up to 54mm (representing 92% increase) compared to a 28mm ball which has been the “standard size” used in the majority of THA. The reasons for improved stability are the following:

1. the large size of the ball (this is the most important feature of the new system for improving instability); and
2. the socket wall is thin (which means the tether provided by the joint capsule is more effective because it is closer to the ball, thereby increasing stability ⁸).

This combination of large ball and thin socket provides increased range of motion without impingement in all planes - flexion, arc of rotation, and ab/adduction. Therefore, the three most important risks of dislocation are addressed:

1. impingement,
2. soft tissue laxity, and
3. errors in component position that tend to be forgiven because of the larger ball, impingement minimization, and improved capsular stability.

As the result of the stability achieved by utilizing the big head with a thin walled socket, we no longer place any restrictions on patient activity after the joint capsule has healed (approximately 6 weeks). They do not have to fear dislocation. They can participate in sporting activities such as yoga and martial arts that ordinarily would place the patient at high risk of dislocation if the typical small ball were implanted.

References for Amstutz Surgeons Perspective

1. Amstutz, H.C. Kody, MH: Dislocation and Subluxation. In Amstutz HC (ed). Hip Arthroplasty. New York, Churchill Livingstone. 429-448, 1991.
2. Amstutz, H.C. Lodwig, R.M. Schurman DJ, Hodgson AG: Range of Motion Studies for Total Hip Replacements. A Comparative Study with a New Experimental Apparatus. Clin Orthop 124-130, 1975.
3. Amstutz HC, Markolf KL: Design Features in Total Hip Replacement. In Harris WH (ed). The Hip Society. St. Louis, 111-124, 1974.
4. Beaule P, Schmalzried T, Udomkiat P, Amstutz H: Jumbo Femoral Head for the Treatment of Recurrent Dislocation Following Total Hip Replacement. Journal of Bone and Joint Surgery 84-A:256-263, 2002.
5. Callaghan JJ, Heithoff BE, Goetz DD, et al: Prevention of Dislocation after Hip Arthroplasty: Lessons from Long-term Followup. Clin Orthop 393:157-162., 2001.
6. Grigoris P, Grecula MJ, Amstutz HC: Tripolar Hip Replacement for Recurrent Prosthetic Dislocation. Clin Orthop 148-155, 1994.
7. Kabo JM, Gebhard JS, Loren G, Amstutz HC: In Vivo Wear of Polyethylene Acetabular Components. J Bone Joint Surg Br 75:254-258, 1993.
8. Kelley SS, Lachiewicz PF, Hickman JM, Paterno SM: Relationship of Femoral Head and Acetabular Size to the Prevalence of Dislocation. Clin.Orthop. 355:163-170, 1998.
9. Phillips C, Barrett J, Losina E, et al: Incidence Rates of Dislocation, Pulmonary Embolism, and Deep Infection During the first six months after Elective Total Hip Replacement. Journal of Bone and Joint Surgery 85-A:20-26, 2003.
10. Von Knoch M, Berry D: Late Dislocation after Total Hip Arthroplasty. Journal of Bone and Joint Surgery 84-A:1949-1953, 2002.