Structured Oral Hip Examination Question 6

EXAMINER: This is an anteroposterior (AP) radiograph of a 72year-old male who had a cemented THA performed 17 years ago. (Figure 2.11.)

CANDIDATE: The AP radiograph demonstrates a cemented THA suggestive of a Stanmore implant. The cup looks worn with the femoral head eccentric in the polyethylene socket. There are lucencies in all three DeLee and Charnley acetabular zones. There is a continuous radiolucency at the femoral cement– bone interface.

EXAMINER: What do we mean by the term wear?

CANDIDATE: Wear is defined as a progressive loss of bearing surface from a material as a result of chemical (corrosive) or mechanical action. Types of mechanical wear include adhesive, abrasive and fatigue.

EXAMINER: What exactly do you mean by abrasive and adhesive wear?

CANDIDATE: Abrasive wear occurs when two surfaces with microscopic irregularities or asperities slide past one another while in intimate contact. The interaction generates particles mainly from the softer material.

Adhesive wear occurs when two opposing materials bond under contact load. Actual transfer of material from one surface to the other may occur, forming transfer films. When motion resumes between the two surfaces, particles may be broken free from one or both surfaces. These new

Figure 2.11 Anteroposterior (AP) radiograph of a loose cemented left THA.

particles then further contribute to wear from third-body abrasive wear.

The wear of ultra-high-molecular-weight polyethylene (UHMWPE) in THA is mainly adhesive and abrasive.

EXAMINER: What is fretting wear?

CANDIDATE: Fretting occurs with small cyclic motions of one surface relative to another.

EXAMINER: What are the wear sources in joint replacement surgery?

CANDIDATE: Wear sources include the primary articulation surface, secondary articulation surfaces, cement/prosthesis micromotion, cement/bone or prosthesis/bone micromotion and third-body wear.

EXAMINER: What are the modes of wear in joint replacement surgery?

CANDIDATE: There are four modes of wear.

1.      Mode 1 is the generation of wear debris that occurs with motion between the two bearing surfaces as intended by the designers.

2.      Mode 2 refers to a primary bearing surface rubbing against a secondary surface in a manner not intended by the designers (for example, a femoral head articulating with an acetabular shell following wear-through of the polyethylene).

3.      Mode 3 refers to two primary bearing surfaces with interposed third-body particles (such as bone, cement, metal and so on).

4.      Mode 4 refers to two non-bearing surfaces rubbing together (such as back-sided wear of an acetabular liner, fretting of the Morse taper, stem–cement fretting).

While several modes of wear often occur simultaneously, mode 1 accounts for the majority of wear in well-functioning hip or knee replacements.

EXAMINER: What do we mean by effective joint space?

CANDIDATE: Schmalzreid et al. coined the term ‘effective joint space’ to refer to all periprosthetic regions to which joint fluid, and hence wear debris, can gain access.¹ In the acetabulum, wear debris can reach the interface through unfilled screw holes or via non-ingrown areas of the shell. On the femoral side, use of circumferential porous coating has reduced the incidence of diaphyseal osteolysis by blocking access of wear particles.

EXAMINER: What is osteolysis?

CANDIDATE: Osteolysis is a biological phenomenon that can result in the loosening of the implant principally caused by the UHMWPE wear particles. Metal or ceramic wear particles that are produced at the articulating surfaces of a hip prosthesis are also implicated but to a much lesser degree. Osteolysis is influenced by the size and morphology of the

UHMWPE particles. Macrophages actively phagocytose (engulf) wear debris at the bone–implant interface. These cells release various enzymes and osteolytic mediators such as interleukin, tumour necrosis factor (TNF-a), and prostaglandin. These cytokines cause inflammation and trigger bone dissolution or resorption around the implanted region.

EXAMINER: What factors influence osteolysis (wear)?

CANDIDATE: Osteolysis (wear) is a multifactorial process dependent on surgical factors, implant design, patient factors and material composition.

Implant-specific factors that affect wear performance of THA

(and TKA) are given in Table 2.7.^2,3

Surgical factors (e.g. component position, soft tissue balancing) that affect joint loads and kinematics which influence wear performance of THA (and TKA) are given in Table 2.8.

Patient-specific factors that affect wear performance of THA (and TKA) are given in Table 2.9.

EXAMINER: What do you know about osteoblastic regulators?

CANDIDATE: Three osteoblastic regulators (RANK, RANKL and OPG) are involved in bone resorption. This is linked to TNF-a, Table

2.7 Implant-specific factors affecting joint wear.

a cytokine responsible for encouraging osteolysis through the facilitation and augmentation of osteoclast differentiation and activation of pre-existing osteoclasts.

Gold medal

Periprosthetic osteolysis is the loss of bone surrounding an artificial implant. The formation of a periprosthetic interfacial membrane between the bone and the implant is implicated in bone resorption. The interfacial membrane is composed primarily of two cell types, the macrophage and the fibroblast.

Aseptic osteolysis is thought to occur through a mechanism involving expression of bone resorptive cytokines such as interleukin-1b (IL-1b), interleukin-6 (IL-6), tumour necrosis factor-a (TNF-a), plateletderived growth factor (PDGF) and receptor activator of nuclear factor-k B ligand (RANKL).

Table 2.8 Surgical factors affecting joint wear.

Table 2.9 Patient-specific factors affecting joint wear.

RANKL is a potent bone resorptive cytokine present on the membranes of bone marrow stromal cells, osteoblasts in bone, as well as on T-cells, and as a soluble molecule secreted into the bone microenvironment by these cells. Receptor activator of nuclear factor-k B (RANK), a RANKL receptor, is expressed on the cell surface of preosteoclasts.

Macrophages express RANK and, when exposed to RANKLin the presence of macrophage colony-stimulatingfactor(M-CSF),havebeenshowntodifferentiateinto matureosteoclastscapableofboneresorption.Osteoprotegerin (OPG) acts as a decoy receptor for RANKL by binding to RANKL and preventing the functional interaction of RANKL with RANK thereby blocking the osteoclast formation and the bone resorptive effects of RANKL. Osteoclast activation is thus blocked.

EXAMINER: What factors affect PE cup wear in THA?

CANDIDATE: Implant factors associated with an increased wear rate include non-cross linked PE, longer shelf-life for liners g-irradiated in air, thickness of PE.

Patient factors include younger age due to higher activity levels, obesity due to increased joint loading. Surgeon factors include position of the cup relative to Kohler’s line, increase in cup abduction angle.

EXAMINER: What is the current thinking about UHMWPE?

CANDIDATE: Three approaches are currently being investigated in an attempt to modify highly cross-linked UHMWPE so that the increased wear resistance provided by cross-linking can be maintained without the reduced fracture resistance that accompanies cross-linking.⁴

1.      Stabilization of free radicals through the impregnation of irradiated ultra-high molecular weight polyethylene with vitamin E. Vitamin E protects polyethylene against oxidation, which renders the melting step that normally follows cross-linking with radiation unnecessary. Vitamin E also quenches free radicals.

2.      A second approach involves sequentially irradiating and annealing polyethylene. Irradiation is conducted in three steps with an interspersed annealing processes that together improve oxidative stability compared with that resulting from a single large dose of irradiation followed by annealing.

3.      The third approach involves the photo-induced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) onto crosslinked polyethylene (CLPE). The concept is to create a hydrophilic layer with better wettability than a conventional polyethylene surface, thus increasing the chance for lubrication.

Endnotes

1.        Schmalzreid TP, Jasty M, Harris WH. Periprosthetic bone loss in total hip arthroplasty: polyethylene wear debris and the concept of the effective joint space. J Bone Joint Surg Am 1992;74-A:849–863. This is a classic hip paper that you need

to know for the exam. With a classic paper ask yourself why the paper is important and how has it changed orthopaedic practice.

2.        Tsao AK, Jones LC, Lewallen DG. What patient and surgical factors contribute to implant wear and osteolysis in total joint arthroplasty? J Am Acad Orthop Surg 2008;16:S7–S13.

3.        For ease of learning and memorizing we have provided the information in table form. Be aware of the need to carefully apply this knowledge into an appropriate usable answer in the exam. If the radiograph demonstrates a malaligned THA (cup open or stem in varus etc.) tell this to the examiners as a probable cause of accelerated wear and then follow up with other surgeon-related factors. Be proactive and mention this sooner rather than later, especially if the topic is travelling down the wear rather than revision route. If the patient is young mention patient-related factors associated with wear such as activity or diagnosis (AVN).

4.        Ramage SC, Urban NH, Jiranek WA, Maiti A, Beckman MJ. Expression of RANKL in osteolytic membranes: association with fibroblastic cell markers. J Bone Joint Surg Am

2007;89-A(4):841–848.

Structured Oral Hip Examination Question 6

EXAMINER: This is an anteroposterior (AP) radiograph of a 72year-old male who had a cemented THA performed 17 years ago. (Figure 2.11.)

CANDIDATE: The AP radiograph demonstrates a cemented THA suggestive of a Stanmore implant. The cup looks worn with the femoral head eccentric in the polyethylene socket. There are lucencies in all three DeLee and Charnley acetabular zones. There is a continuous radiolucency at the femoral cement– bone interface.

EXAMINER: What do we mean by the term wear?

CANDIDATE: Wear is defined as a progressive loss of bearing surface from a material as a result of chemical (corrosive) or mechanical action. Types of mechanical wear include adhesive, abrasive and fatigue.

EXAMINER: What exactly do you mean by abrasive and adhesive wear?

CANDIDATE: Abrasive wear occurs when two surfaces with microscopic irregularities or asperities slide past one another while in intimate contact. The interaction generates particles mainly from the softer material.

Adhesive wear occurs when two opposing materials bond under contact load. Actual transfer of material from one surface to the other may occur, forming transfer films. When motion resumes between the two surfaces, particles may be broken free from one or both surfaces. These new

Figure 2.11 Anteroposterior (AP) radiograph of a loose cemented left THA.

particles then further contribute to wear from third-body abrasive wear.

The wear of ultra-high-molecular-weight polyethylene (UHMWPE) in THA is mainly adhesive and abrasive.

EXAMINER: What is fretting wear?

CANDIDATE: Fretting occurs with small cyclic motions of one surface relative to another.

EXAMINER: What are the wear sources in joint replacement surgery?

CANDIDATE: Wear sources include the primary articulation surface, secondary articulation surfaces, cement/prosthesis micromotion, cement/bone or prosthesis/bone micromotion and third-body wear.

EXAMINER: What are the modes of wear in joint replacement surgery?

CANDIDATE: There are four modes of wear.

1.      Mode 1 is the generation of wear debris that occurs with motion between the two bearing surfaces as intended by the designers.

2.      Mode 2 refers to a primary bearing surface rubbing against a secondary surface in a manner not intended by the designers (for example, a femoral head articulating with an acetabular shell following wear-through of the polyethylene).

3.      Mode 3 refers to two primary bearing surfaces with interposed third-body particles (such as bone, cement, metal and so on).

4.      Mode 4 refers to two non-bearing surfaces rubbing together (such as back-sided wear of an acetabular liner, fretting of the Morse taper, stem–cement fretting).

While several modes of wear often occur simultaneously, mode 1 accounts for the majority of wear in well-functioning hip or knee replacements.

EXAMINER: What do we mean by effective joint space?

CANDIDATE: Schmalzreid et al. coined the term ‘effective joint space’ to refer to all periprosthetic regions to which joint fluid, and hence wear debris, can gain access.¹ In the acetabulum, wear debris can reach the interface through unfilled screw holes or via non-ingrown areas of the shell. On the femoral side, use of circumferential porous coating has reduced the incidence of diaphyseal osteolysis by blocking access of wear particles.

EXAMINER: What is osteolysis?

CANDIDATE: Osteolysis is a biological phenomenon that can result in the loosening of the implant principally caused by the UHMWPE wear particles. Metal or ceramic wear particles that are produced at the articulating surfaces of a hip prosthesis are also implicated but to a much lesser degree. Osteolysis is influenced by the size and morphology of the

UHMWPE particles. Macrophages actively phagocytose (engulf) wear debris at the bone–implant interface. These cells release various enzymes and osteolytic mediators such as interleukin, tumour necrosis factor (TNF-a), and prostaglandin. These cytokines cause inflammation and trigger bone dissolution or resorption around the implanted region.

EXAMINER: What factors influence osteolysis (wear)?

CANDIDATE: Osteolysis (wear) is a multifactorial process dependent on surgical factors, implant design, patient factors and material composition.

Implant-specific factors that affect wear performance of THA

(and TKA) are given in Table 2.7.^2,3

Surgical factors (e.g. component position, soft tissue balancing) that affect joint loads and kinematics which influence wear performance of THA (and TKA) are given in Table 2.8.

Patient-specific factors that affect wear performance of THA (and TKA) are given in Table 2.9.

EXAMINER: What do you know about osteoblastic regulators?

CANDIDATE: Three osteoblastic regulators (RANK, RANKL and OPG) are involved in bone resorption. This is linked to TNF-a, Table

2.7 Implant-specific factors affecting joint wear.

a cytokine responsible for encouraging osteolysis through the facilitation and augmentation of osteoclast differentiation and activation of pre-existing osteoclasts.

Gold medal

Periprosthetic osteolysis is the loss of bone surrounding an artificial implant. The formation of a periprosthetic interfacial membrane between the bone and the implant is implicated in bone resorption. The interfacial membrane is composed primarily of two cell types, the macrophage and the fibroblast.

Aseptic osteolysis is thought to occur through a mechanism involving expression of bone resorptive cytokines such as interleukin-1b (IL-1b), interleukin-6 (IL-6), tumour necrosis factor-a (TNF-a), plateletderived growth factor (PDGF) and receptor activator of nuclear factor-k B ligand (RANKL).

Table 2.8 Surgical factors affecting joint wear.

Table 2.9 Patient-specific factors affecting joint wear.

RANKL is a potent bone resorptive cytokine present on the membranes of bone marrow stromal cells, osteoblasts in bone, as well as on T-cells, and as a soluble molecule secreted into the bone microenvironment by these cells. Receptor activator of nuclear factor-k B (RANK), a RANKL receptor, is expressed on the cell surface of preosteoclasts.

Macrophages express RANK and, when exposed to RANKLin the presence of macrophage colony-stimulatingfactor(M-CSF),havebeenshowntodifferentiateinto matureosteoclastscapableofboneresorption.Osteoprotegerin (OPG) acts as a decoy receptor for RANKL by binding to RANKL and preventing the functional interaction of RANKL with RANK thereby blocking the osteoclast formation and the bone resorptive effects of RANKL. Osteoclast activation is thus blocked.

EXAMINER: What factors affect PE cup wear in THA?

CANDIDATE: Implant factors associated with an increased wear rate include non-cross linked PE, longer shelf-life for liners g-irradiated in air, thickness of PE.

Patient factors include younger age due to higher activity levels, obesity due to increased joint loading. Surgeon factors include position of the cup relative to Kohler’s line, increase in cup abduction angle.

EXAMINER: What is the current thinking about UHMWPE?

CANDIDATE: Three approaches are currently being investigated in an attempt to modify highly cross-linked UHMWPE so that the increased wear resistance provided by cross-linking can be maintained without the reduced fracture resistance that accompanies cross-linking.⁴

1.      Stabilization of free radicals through the impregnation of irradiated ultra-high molecular weight polyethylene with vitamin E. Vitamin E protects polyethylene against oxidation, which renders the melting step that normally follows cross-linking with radiation unnecessary. Vitamin E also quenches free radicals.

2.      A second approach involves sequentially irradiating and annealing polyethylene. Irradiation is conducted in three steps with an interspersed annealing processes that together improve oxidative stability compared with that resulting from a single large dose of irradiation followed by annealing.

3.      The third approach involves the photo-induced graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) onto crosslinked polyethylene (CLPE). The concept is to create a hydrophilic layer with better wettability than a conventional polyethylene surface, thus increasing the chance for lubrication.

Endnotes

1.        Schmalzreid TP, Jasty M, Harris WH. Periprosthetic bone loss in total hip arthroplasty: polyethylene wear debris and the concept of the effective joint space. J Bone Joint Surg Am 1992;74-A:849–863. This is a classic hip paper that you need

to know for the exam. With a classic paper ask yourself why the paper is important and how has it changed orthopaedic practice.

2.        Tsao AK, Jones LC, Lewallen DG. What patient and surgical factors contribute to implant wear and osteolysis in total joint arthroplasty? J Am Acad Orthop Surg 2008;16:S7–S13.

3.        For ease of learning and memorizing we have provided the information in table form. Be aware of the need to carefully apply this knowledge into an appropriate usable answer in the exam. If the radiograph demonstrates a malaligned THA (cup open or stem in varus etc.) tell this to the examiners as a probable cause of accelerated wear and then follow up with other surgeon-related factors. Be proactive and mention this sooner rather than later, especially if the topic is travelling down the wear rather than revision route. If the patient is young mention patient-related factors associated with wear such as activity or diagnosis (AVN).

4.        Ramage SC, Urban NH, Jiranek WA, Maiti A, Beckman MJ. Expression of RANKL in osteolytic membranes: association with fibroblastic cell markers. J Bone Joint Surg Am

2007;89-A(4):841–848.