Clinical Need

The genesis of Total Hip Arthroplasty (THA) occurred in the late 1950s via an innovative surgeon named Dr. John Charnley. Since the introduction of THA by Dr. Charnley, there has been improvements in both hip implant device concepts and refinements of the initial design.

There are numerous Hip Implant Systems available to surgeons and patients today; however, the respective functional designs remain the same with minimal differentiation. All systems maintain a femoral stem with an attached ball and an acetabular cup component. There are three clearly defined system categories: Conventional, Dual Mobility and Constrained.

Although skilled surgeons have optimized existing Hip Implant System offerings, significant challenges associated with THA remain. Primary complications are dislocation and instability, component positioning / placement and edge loading.

Dislocation and Instability

The most significant THA complications causing the greatest physician concern, increased health care costs and reduced patient satisfaction are dislocation and instability. Simply described, instability is the inability to maintain a reduced joint and the hip lacks complete stability. Implant dislocation occurs with a traditional hip implant system when the ball-shaped head of the femoral stem dislodges from the acetabular cup.

Recurrent instability after primary and revision surgeries is a significant issue with current hip replacement systems. Certain component positions can cause the ball of the hip prosthesis to sublux and/or dislocate. To avoid this, patients are sometimes advised after surgery not to bend more than 90 degrees at the hip and to not let their leg cross the midline of the body. The risk for dislocation is greatest in the first few months after surgery while the tissues are healing.

Component Positioning / Placement

Accurate positioning of implant components during total hip arthroplasty is a challenging and critical step in hip replacement. Placement of the acetabular cup is of great importance since nearly every deviation from the ideal center of rotation negatively influences endoprosthesis survival, polyethylene wear and hip load. Back in 1978, Lewinnek, et al found that the dislocation rate was significantly higher when the acetabular cup was placed outside of a "safe zone", defined as 15 ± 10 degrees anteversion and a lateral opening of 40 ± 10 degrees.¹ Acetabular cup orientation has been shown to influence dislocation, impingement, edge loading, contact stress, and polyethylene wear in total hip arthroplasty.^2,3,4,5 Numerous studies have demonstrated that correct acetabular cup position is critical to successful total hip replacement. Unfortunately, malposition of acetabular cups is quite common. In some cases it may be unavoidable due to anatomical limitations that can affect cup placement (e.g. hip dysplasia, obesity, etc.).

Edge Loading

Edge loading is a chiseling effect that can occur when the ball of the implant presses on the socket edge. Edge loading is associated with acetabular cup malposition and may cause accelerated component wear and increased debris particles that heighten the likelihood of osteolysis and subsequent loosening and failure of the implant.

Unfortunately, all of these potential complications associated with THA can be serious.

¹ Lewinnek, G., Lewis, J.L., et al (1978). "Dislocations after Total Hip-Replacement Arthroplasties." J Bone Joint Surg Am. 60:217-220.
² Gallo, J., V. Havranek, et al. (2010). "Risk factors for accelerated polyethylene wear and osteolysis in ABG I total hip arthroplasty." Int Orthop 34(1): 19-26.
³ Bicanic, G., D. Delimar, et al. (2009). "Influence of the acetabular cup position on hip load during arthroplasty in hip dysplasia." Int Orthop 33(2): 397-402.
⁴ Gray, C. F., R. E. Moore, et al. (2012). "Spontaneous dissociation of offset, face-changing polyethylene liners from the acetabular shell: a report of four
   cases." J Bone Joint Surg Am 94(9): 841-845.
⁵ Bicanic, G., D. Delimar, et al. (2009). "Influence of the acetabular cup position on hip load during arthroplasty in hip dysplasia." Int Orthop 33(2): 397-402.