Bi-Metric stem with metal-metal articular couple enhances stability

Issue: June 2006

ByJohn M. Cuckler, MD

John M. Cuckler, MD [photo]
John M. Cuckler

The Bi-Metric stem (Biomet, Orthopedics, Inc., Warsaw, Ind.) is a tapered, proximally coated (plasma-sprayed, commercially pure [CP]) titanium femoral stem available in both standard and lateral offset designs, and with or without a collar. Tapered stems have a long history of successful clinical performance.

The Bi-Metric stem uses a 3° biplanar continuous taper design to achieve excellent initial stability (Figure 1). In vitro testing shows the tapered stem demonstrates initial stability equivalent to that of cemented stems.¹ Plasma-spray porous coating has the advantage of excellent early stability due to its high frictional interface with adjacent bone and closed-pore structure to prevent particulate debris migration.² A further advantage of plasma-spray coatings, as compared with other porous coatings, is that up to 90% of the fatigue strength of the femoral stem is preserved. The Bi-Metric stem has plasma-spray coating over the proximal 25% of the stem; the distal stem has a grit-blasted Interlok (Biomet Orthopedics, Inc., Warsaw, Ind.) finish for enhanced stability and bone ongrowth.

Figure 1. The Bi-Metric stem is a 3º biplanar, taper design with proximal porous coating.

Source: Cuckler JM

Advantages and clinical results

An advantage of the tapered stem is its simplicity of insertion. Distal reaming of the femoral canal is optional but not necessary. An important advance in the precision of the Bi-Metric stem was the introduction of the “Exact-Fit” technology, which produces stems and broaches of identical exterior dimensions, allowing precise restoration of hip anatomy and limb length.

The addition of lateral offset stems optimizes the hip abductor function and hip anatomy. The design of the prosthetic femoral neck allows lateralization of the stem axis without lengthening of the extremity. Lateral offset stems are most commonly used in patients who are tall or in patients with medialization of the center of rotation of the acetabulum.

An additional design feature is progressive anatomic growth of the stem dimensions with increasing stem size. The stem is available in 1-mm incremental sizes from 7 mm to 21 mm. Length of the stem increases from 115 mm for the smallest stem to 185 mm for the largest stem. Offset increases from 39 mm to 43 mm for the standard offset stems as the implants increase in size, and the lateral offset stems increase offset from 44 mm to 52 mm.

The tapered stem design of the Bi-Metric femoral component is significantly forgiving. Since I began using the Bi-Metric stem nine years ago, I have not made a single revision. Early weight-bearing has not produced failure of fixation or subsidence. Slight varus positioning of the stem or undersizing does not appear to compromise stem stability. Thigh pain has been nonexistent with this design. Ten-year follow-up of the Bi-Metric stem is reported to be between 99.3% and 100%.^3-5

Metal-metal articulation

The metal-metal total hip arthroplasty has a clinical history longer than any available wear couple and has demonstrated clinical results equal to or better than metal-polyethylene hip arthroplasties. In contrast with metal-polyethylene hips, larger diameter metal-metal femoral heads demonstrate superior wear behavior in comparison with smaller diameter heads. Large diameter femoral heads can safely be used to enhance stability of the hip arthroplasty, which is unique among available materials combinations for hip replacement.

The wear process of the metal-metal total hip replacement (THR) produces submicron metal particulates, in comparison with metal-polyethylene wear. The wear process liberates the constituent ions of cobalt and chrome, which are excreted primarily in the urine. In general, ion elevations three times to five times higher than control levels are reported, although these levels vary with factors such as activity level or articular damage, such as that which occurs with dislocation. Renal failure can produce exceedingly high ion levels and is generally considered a relative contraindication to using the metal-metal THR. Although one report indicates the ions may not cross the maternal placental membrane, there is insufficient data available at this time to assess the safety of the metal-metal THR to the fetus. Careful deliberation and discussion of this issue is appropriate for women of childbearing age considering a metal-metal THR.

The long-term effects of ion exposure seem benign based on past experience with early metal-metal designs and current experience among approximately 400,000 recipients of second-generation metal-metal THRs. As with all long-term medical implants, continued follow-up and monitoring of arthroplasty patients will be the ultimate determinant of the physiologic effects of the wear process.

THR advances with larger femoral heads

A significant advance in total hip arthroplasty is the advent of the large femoral heads possible with metal-metal arthroplasty. The M2a-Magnum (Biomet Orthopedics, Inc., Warsaw, Ind.) system offers nonmodular cast cobalt chrome acetabular components with CP plasma spray coating in diameters from 44 mm to 66 mm, in 2-mm increments (Figure 2). The cobalt chrome femoral heads used with these sockets vary from 38 mm to 60 mm in diameter and are 6 mm smaller than the outside diameter of the acetabular component.

Figure 2. The Magnum system uses a nonmodular acetabular component of plasma-spray-coated, cast cobalt chrome, with a modular cobalt chrome femoral head. The head insert achieves variation in head length from -6 mm to +9 mm.

Source: Cuckler JM

The large femoral head diameter allows up to 162° range of motion without impingement, thus significantly enhancing stability of the hip. In addition, the large femoral head diameter increases the hop height of the femoral head over the acetabular component up to 2.2 cm, thus increasing dislocation resistance.

A prospective, randomized study of the clinical results of metal-metal total hip arthroplasty showed no differences between conventional 28-mm polyethylene arthroplasties and 28-mm metal-metal THRs at intermediate follow-up.⁶

A clinical study was performed comparing the incidence of dislocation between 28-mm and 38-mm femoral heads within the first three months after hip arthroplasty.⁷ The incidence of dislocation among 78 hip replacements using 28-mm metal-metal femoral heads was 2%, while no dislocations were observed (P < .0001) among 634 femoral heads 38-mm in diameter. This clinical experience allows me to relax the normal hip precautions, thus allowing rapid recovery for the patient.

Regarding clinical results and the absence of osteolysis, the Bi-Metric stem demonstrates significant success at follow-up of up to 15 years. The addition of the metal-metal articular couple now offers surgeons and patients the prospect of long-term success with the additional advantage of improved stability and resistance to dislocation.

References

Sharkey PF, Albert TJ, Hume EL, Rothman RH. Initial stability of a collarless wedge-shaped prosthesis in the femoral canal. Semin Arthroplasty. 1990;1:87-90.
Bourne RB, Rorabeck CH, Burkart BC, Kirk PG. Ingrowth surfaces. Plasma spray coating to titanium alloy hip replacements. Clin Orthop. 1994;298:37-46.
Varjonen S, Lepistö J, Alho A. Cementless hip arthroplasty - Ten years follow-up. Third Domestic Congress of the European Hip Society. June 1998.
Meding JB. Minimum ten-year follow-up of a straight-stemmed, plasma-sprayed, titanium-alloy, uncemented femoral component. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; Feb. 28 – March 4, 2001; San Francisco.
Evans J. Outcome of a tapered, titanium, proximal load-bearing, non-cemented femoral THA component: A minimum 5-year follow-up study. Presented at: American Academy of Orthopaedic Surgeons Annual Meeting; March 19–23, 1998; New Orleans.
Lombardi AV Jr, Mallory TH, Cuckler JM, et al. Mid-term results of a polyethylene-free metal-on-metal articulation. J Arthroplasty. 2004;19:42-47.
Cuckler JM, Moore KD, Lombardi AV Jr, et al. Large versus small femoral heads in metal-on-metal total hip arthroplasty. J Arthroplasty. 2004;19:41-44.

John M. Cuckler, MD, practices in the division of orthopedics at the University of Alabama at Birmingham