New for large head THA: Anatomic mobile-bearing hip system

New system offers good stability, wear and ROM.

David C. Markel

A new anatomic mobile-bearing hip system addresses key concerns, such as stability and wear potential, as well as malpositioning problems and motion, related to performing total hip arthroplasty (THA) with large head, metal-on-metal implants, according to David C. Markel, MD.

Large-head implants are popular, said Markel, chief of orthopedics at Providence Hospital, Southfield, Mich. They offer the potential for greater range of motion and stability. Additionally, through increased jump height, they are able to address the risk of dislocation, one of the primary complications related to total hip replacement (THR).¹ Large heads give “everyone a feeling of protection,” Markel said.

Although THA has been a successful operation, there are issues, particularly with primary metal-on-metal implants. Based on data from the Australian Hip Registry,² there is growing concern about metal ion release and early failures.

The ADM X3 Mobile Bearing Hip System (Stryker Orthopaedics) is designed to address many of these concerns. Although similar dual mobility designs have been used for many years in France, the ADM system was only recently introduced in the United States.

At first, Markel was unsure about the ADM. “When first introduced to it, I was intrigued by the large head, thick polyethylene and dual mobility articulation,” Markel said.

Anatomic design

The ADM shell design features an anatomic rim that incorporates a psoas cutout to help provide relief between the acetabular shell rim and the iliopsoas tendon. A dual mobility construct with a 28-mm inner head (metal or ceramic) and a large outer polyethylene insert articulates within this shell. This design allows for a 6-mm difference between the outer shell and the large diameter insert. Therefore, in most primary situations, the large outer head measures in the 46 mm to 50 mm range. These large heads permit greater range of motion without impingement and a large jump height.³

“The advantage is that there is no chipping, there is no edge load effect, and we have clearly avoided the ion issues.”

— David C. Markel, MD

The inner bearing accommodates most of the hip motion, whereas the outer bearing will accommodate the extreme ranges of motion. This dual motion unlinks some of the cross shear, which potentially further decreases wear potential,⁴ Markel said.

Good wear comparisons

Simulator testing demonstrated dramatic wear results, he said. “When we looked at it early, everyone was impressed that this dual motion mechanism was bringing the wear rate down,” Markel said.

Perhaps even more noteworthy is the testing of this implant at a high abduction angle. At 65· of inclination, ADM X3 showed a 94% reduction in wear compared with metal-on-metal (Figure 1).⁵ “Even in these conditions, the wear rates are immeasurably low,” he said. “That may be because the head can reorient a bit with the outer polyethylene bearing and help the surgeon avoid any edge loading.”

“The advantage is that there is no chipping, there is no edge load effect, and we have clearly avoided the ion issues,” Markel said. “When we look at range of motion and jump height, that may be the most impressive thing with this bearing.”

The implant uses a standard femoral stem and a unique anatomic cup design.

The bearing provides a large arc of motion, but generally not as large as many metal-on-metal offerings. Most metal-on-metal bearings have cups that are less than a hemisphere allowing for larger ranges of motion before impingement. The trade-off is high-angled edge loading and loss of jump height.^3,6 The ADM “is more than a hemisphere in some areas due to its anatomic design,” he said. Laboratory testing has shown that this provides more than three times greater jump distance than competitive hard-on-hard bearings (Figure 2).³

Unique appearance

The ADM has a unique appearance on postoperative radiographs. “When you look at this cup, it does not look like a traditional acetabular component,” Markel said. Cutouts and some overhang are a result of the anatomic shell. “It is important to look at the schematic by drawing lines based on where the anatomic cutouts are located. When you put it in anatomically, it has a tremendous mechanical fit.”

Overall, the ADM appears to address all of the goals for larger head THA, Markel said. “It has shown very good wear. At this point, based on the clinical experience in France, it looks like it has definite longevity, good range of motion and good stability.”⁷

References

1. Berry DJ, et al. Effect of femoral head diameter and operative approach on risk of dislocation after primary THA. J Bone Joint Surg. 2005;87-A(11):2456-2463.
2. 2010 Annual Report, AOANJRR (Australian Orthopaedic Association National Joint Replacement Registry). http://www.dmac.adelaide.edu.au/aoanjrr/documents/Identified_Prostheses/Primary_Hip/Primary_Total_Hip/Total_Conventional/2@~Re-identified_and_still_used/ASR_Acetabular_Prosthesis.pdf.
3. Nevelos J, et al. Acetabular bearing design has a greater influence on jump distance than head size. Poster #2028. Presented at the 56th Annual Meeting of the Orthopaedic Research Society. March 6-9, 2010. New Orleans.
4. Adam P, Farizon F, Fessy MH. Dual articulation retentive acetabular liners and wear: surface analysis of 40 retrieved polyethylene implants. Rev Chir Orthop Reparatrice Appar Mot. 2005;91:627-636.
5. Herrera L, Lee R, Longaray J, et al. Edge loading wear due to inclination angle for three contemporary hip bearings. Poster #2259. Presented at the 56th Annual Meeting of the Orthopaedic Research Society. March 6-9, 2010. New Orleans.
6. Griffin W, Nanson C, Springer G, et al. Reduced articular surface of one-piece cups: A cause of runaway wear and early failure. Clin Orthop Relat Res. 2010;468(9):2328-2332.
7. Philippot R, Camilleri JP, Boyer B, Adam P, Faizon F. The use of a dual-articulation acetabular cup system to prevent dislocation after primary total hip arthroplasty: Analysis of 384 cases at a mean follow-up of 15 years. International Orthopaedics (SICOT). 2009; 33:927-932.

David D. Markel, MD, can be reached at the Porretta Center for Orthopaedic Surgery, 22250 Providence Drive, Suite 401, Southfield, MI 48075; (248) 349-7015; email: dmarkel@providence-hospital.org.