Reconstruction Cases Bearing Materials And Associated Complications

An 82-year-old woman, with a BMI of 33.9 and history of total hip replacement performed 8 years ago, sustained a fall when attempting to walk to the bathroom. She has been unable to bear weight or ambulate since the fall. Radiographs and clinical picture of the implant in Figure 7–1 reveal a fracture of the femoral neck. This mechanism of implant failure likely occurred due to:

Figure 7–1

 

Based on biomechanical literature available, this fracture occurred due to:

1. Mismatch between the tensile strength of the stem and neck
2. Mechanical preparations and markings on the implant
3. Nonmodular nature of the component
4. Trunnionosis and interface wear
5. The increased interface stresses of cemented implants

 

Discussion

The correct answer is (B). This patient had a hybrid total hip arthroplasty with a

cemented femoral component and a press-fit acetabular component. The fracture most likely occurred due to mechanical etching/laser marking on the component during production. Multiple manufacturers have experienced similar failures with designs in the late 1990s and have withdrawn those devices off the market. Fractures of femoral stems at the modular head–neck junction have also been reported infrequently and are due to fretting fatigue. Risk factors for such fractures include: excessive body weight and inadequate proximal osseous support because of trochanteric osteotomy, reduced preoperative bone stock, osteolysis, loosening, and/or implant undersizing.

“Trunnionosis” is the phenomenon of wear debris generated at the modular junction between the femoral head and tapered femoral neck (“trunnion”) of the stem. Implant generated metallic wear debris can be a concern when large femoral head sizes are used (greater than 40 mm in diameter) as well as metal-on-metal bearing surfaces. However in this case, radiographs demonstrate that a 32-mm head was used, and there was no clear indication of wear. The bearing surface in this implant is metal on polyethylene.

During cement mixing, the process of polymerization is started by:

1. Barium sulfate
2. Benzoyl peroxide
3. Dimethyl toluidine
4. Hydroquinone
5. Hylamer

 

Discussion

The correct answer is (B). Poly(methyl methacrylate) (PMMA) is a transparent Plexiglas typically used in joint arthroplasty. The material acts as a grout to solidify the interface between bone and the implant. During mixing, a powder of polymer, barium sulfate, and an initiator is combined with the liquid monomer and accelerator. Benzoyl peroxide is typically used as an initiator. Hydrogen peroxide can play a similar role. Dimethyl toluidine is a typical accelerator. Barium sulfate is usually used in cements to make the material radiodense and visible on x-rays.

Modern cement techniques have advanced the preparation and application of PMMA. First-generation cementing involved finger packing the cement, which is still the method often used on the acetabular side. The difference between third generation and second generation usually involves the use of:

1. Pressurization
2. Cement guns
3. Pulse lavage and drying the canal
4. Brushing the canal
5. Cement restrictor

 

Discussion

The correct answer is (A). Second-generation techniques introduced cement guns, restrictors, as well as better techniques of preparing the canal by pulse lavage, brushing, and drying. Third-generation techniques usually involve vacuum mixing, precoating the implant surface, and pressurization. The Swedish registry suggested that vacuum mixing may worsen long-term survival. Ideal cementing technique involves at least a 2-mm mantle in all seven Gruen zones. The stem should take at least ⅓ of the canal. The distal mantle should be at least one diameter. Barrack et al. have defined a grading system for cementing with grade A showing a uniform, even mantle around the entire prosthesis while grade D has 100% radiolucency at the interface. The quality of cementing has been shown to directly correlate with outcome.

During preparation the cement goes through multiple stages that include a “doughy” phase, working time, insertion time, and setting time. While setting, cement undergoes an exothermic reaction with 12 to 14 kcal released per 100-g cement. Rapid mixing, high humidity, and high temperature shorten the process.

The press-fit acetabular component is widely accepted as a standard in the United States. The Swedish registry data, though, show that:

1. Press-fit stems generally have less pain and less revision rate
2. Press-fit cups have higher failure rate and revisions
3. Younger patients benefit from press-fit total hip arthroplasty
4. Cemented femurs had a higher rate of fracture
5. In femoral neck fractures, press-fit implants generally have better survival

 

Discussion

The correct answer is (B). The Swedish Hip Arthroplasty Registry is an extensive, detailed National Quality database of outcomes following THR performed in Sweden since 1979. Results from the Swedish Registry data support the use of cemented total hip replacement for both the femur and acetabulum. Press-fit stems have a higher rate of fracture, subsidence, and thigh pain. No specific differences

were seen per age group, even though the revision curve for both cemented and uncemented components seem to decline faster after 10 to 12 years. Some authors suggest that, in the setting of insufficiency fractures that are associated with worse bone quality, cemented total hip replacements provide more reliable results. Press-fit stems, especially with use of aggressively tapered designs, leads to higher risk of fractures.

Osteolysis is seen in Charnley zone 1 around the acetabulum and mild asymmetry in the position of the femoral head in the cup. The process through which osteolysis takes place in this design is driven by:

1. Lymphocytes
2. Macrophages
3. Osteoblasts
4. Eosinophils
5. Lymphoblasts

 

Discussion

The correct answer is (B). Metal-on-polyethylene hips lead to polyethylene particle wear. The subsequent osteolysis is usually driven by a macrophage mediated process. By comparison, the recently described metal particle wear is a lymphocytic process, termed ALVAL (aseptic lymphocyte-dominated vasculitis-associated lesions). Adverse local tissue reactions (ALTR) around metal-on-metal components is being increasingly recognized, initially due to the bearing surface, and more recently due to the process of trunnionosis.

The bearing surface used in this total hip replacement is metal-on-polyethylene. Alternative bearings include metal-on-metal, ceramic-on-ceramic, and ceramic-on-polyethylene. By comparison to first-generation ceramic bearing surfaces, third-generation ceramic-on-ceramic bearings show:

1. Higher incidence of osteolysis, squeaking, and head fractures
2. Higher incidence of pain and instability
3. Lower incidence of osteolysis, squeaking, and head fractures
4. Higher failure rate and need for revision
5. 100% survival at 10 years

 

Discussion

The correct answer is (C). Ceramic bearings have demonstrated significantly better

wear characteristics compared to conventional metal-on-polyethylene bearing surfaces both clinically and in wear simulators. Squeaking, which has been documented previously with initial ceramic designs, is now reported at less than 1%.

 

Helpful Tip:

Both cemented and press-fit fixation techniques are associated with good results following total hip arthroplasty. Cementing technique is critical in determining long-term implant survival and should include canal preparation, pressurization, and a good cement mantle interface.Objectives: Did you learn...?

 

 

How to recognize bearing materials and associated complications? Implant fixation mechanisms and modern cementing techniques?

 

Bearing surface wear and the biologic process involved?