The Effects of Osteolysis and Aseptic Loosening

Medical Writer: Ivan Oransky, MD Editor in chief, Praxis Post ()Host Orthopaedic Surgeon: Raj K. Sinha, MD, PhD Assistant Professor of Orthopaedic Surgery Division of Adult Reconstructive Surgery Co-Director, Ferguson Laboratory Department of Orthopaedic Surgery University of Pittsburgh Medical Center Pittsburgh, PennsylvaniaDr. Marshal Peris Chief Resident, Department of Orthopaedic Surgery University of Pittsburgh Medical Center Pittsburgh, Pennsylvania 

Medscape Orthopedics. 2001;5(1) 

The Effects of Osteolysis and Aseptic Loosening

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Although success rates for total hip arthroplasty (THA) now approach 97%, osteolysis and aseptic loosening continue to plague surgeons. In fact, reported prosthetic failure rates due to these complications are as high as 20%.[1] When cemented femoral components were the predominant types of prostheses used, as many as 12% of patients demonstrated symptomatic loosening, and as many as 20% required revision surgery.[2]

Osteolysis is the end result of a biologic process that begins when the number of wear particles generated in the joint space overwhelms the capsule's capacity to clear them. The residual particles stimulate a macrophage-induced inflammatory response that can lead to bone loss and subsequent implant loosening. Although cement particles were once exclusively blamed for osteolysis, it has become clear that any particle debris can result in bone resorption. For example, in a cemented THA, while it is easy to imagine that a cement mantle defect could allow particles to migrate and infiltrate the canal, what about cases in which there is little wear, and still osteolysis results?

The effects of osteolysis and aseptic loosening were highlighted at an orthopaedic Grand Rounds presentation at the University of Pittsburgh on October 20, 2000, hosted and led by Dr. Raj Sinha, with a presentation by Dr. Marshal Peris.

A 35-year-old man with a history of pigmented villonodular synovitis of the right hip, status post open synovectomy, presents with collapse of the femoral head. Past medical history and medication history are noncontributory, and there is no history of tobacco or alcohol use.

In 1995, the patient underwent primary cemented THA, which was followed by an uneventful postoperative course and no problems over the next 4 years.

In September 1999, the patient began to develop pain in his thigh and groin.

X-rays and a bone scan taken at that time were consistent with loosening of the femoral component.

At that time, the patient was observed for 3 months, but the pain worsened and he was referred for possible revision THA.

As discussed below, the patient then proceeded to revision surgery.

The differential for painful THA includes:

  • acetabular loosening;

  • femoral component loosening;

  • sciatica or lumbar stenosis;

  • trochanteric bursitis;

  • thigh pain and microfracture;

  • infection; and

  • polyethylene wear and osteolysis.

The first step in evaluating a painful THA is to determine whether the pain is indeed related to the prosthesis. Careful attention should be paid to the following factors when obtaining a history:

  • the location of the pain;

  • whether the pain is related to activity;

  • positions that offer pain relief; and

  • the presence of nighttime pain.

The physical examination should include the following:

  • evaluation for signs of bacteremia;

  • palpation of the greater trochanter for tenderness;

  • examination for potential passive range of motion (ROM) pain;

  • lumbar spine exam;

  • evaluation of abductor and flexor strength; and

  • listening for snapping or popping on ambulation.

X-ray criteria for loosening of the femoral component are as follows. For cemented stems:

  • subsidence;

  • cement fracture;

  • fractured stem; and

  • divergent or progressive radiolucency.

For uncemented stems:

  • subsidence;

  • distal pedestal formation (not simply a radiodense line at the tip of a cementless stem, but weight-bearing pedestal)

  • cortical hypertrophy; and

  • divergent radiolucency.

By what mechanism do components fail after loosening? In general terms, mechanical factors initiate femoral failure, while biologic components initiate acetabular failure. In femoral component failure, debonding — the deterioration of the stem-cement bond -- occurs early. The first sign of debonding is subsidence within the cement mantle visible by a radiolucent line at the superolateral aspect of the prosthesis.

Debonding of the stem from the cement seemed to be an important initiating factor in failure of cemented stems. Therefore, increasing the strength of this bond seemed logical. Increasing the surface roughness substantially increased the shear strength of the cement implant interface, but if micromotion does occur, the amount of wear debris generated would also increase dramatically.[4] This is consistent with the massive lysis observed in a recent series of rough-surface cemented stems that debonded early.

However, other series of rough-surface stems, such as the HD2, Spectron, and Lubinus, have not experienced early debonding or osteolysis. It is thought that the surface finish alone does not explain these early, dramatic failures. Early debonding is associated with other suboptimal design parameters, such as a circular cross-sectional geometry and inadequate length.

Some experts believe that debonding initiates the process of aseptic loosening, while others propose that the subsidence of highly polished and smooth components to a stable position is normal. And the literature[3] suggests that debonding of a smooth stem from the cement mantle does not necessarily result in component loosening. In 297 THAs followed for at least 20 years or until revision or death, there was a significantly poorer probability of survival in those components that demonstrated a radiolucent line measuring greater than 2 mm at the superolateral aspect of the component.[3] These conflicting philosophies have led to the development of different prosthetic designs: some are polished and intended to tolerate small amounts of debonding, while others are given surface treatments such as texturing or precoating with methylmethacrylate to maximize initial bonding with the cement.

Debonding often leads to high peak stresses in the cement mantle, which can result in fractures, most of which occur proximally. Once this occurs, the thin mantles, as well as the defects and pores in the cement associated with most mantle fractures, make the implant more susceptible to biologic factors. Once debonding and fracture occur, debris can gain access to the distal endosteal bone, which leads to an immunologic reaction and bone resorption.

Radiographically, osteolysis falls into 2 categories: linear and expansile. Linear osteolysis is characterized by generalized canal enlargement and nonfocal endosteal bone lysis. Expansile osteolysis is typified by uneven or scalloped focal or multifocal endosteal bone lysis.

The osteolytic process varies depending on the type of stem used. In cemented THA, osteolysis can be decreased by improved techniques such as the use of a canal restrictor, pulsatile lavage to remove debris, epinephrine to coagulate the endosteal surface, and pressure injection of the cement. Linear osteolysis is the most common type associated with cemented stems. In the 1970s, 10 years postoperation, surgeons noted that as many as a third of these stems demonstrated loosening.[2] This high rate of loosening was one of the primary motives for the development of uncemented prosthetic systems.

Studies show that osteolysis is also common with first-generation uncemented systems, with 10% to 15% of stems demonstrating loosening at 5-year follow-up.[1] With uncemented stems, expansile osteolysis is more common than linear osteolysis.

Ironically, although loosening of cemented stems prompted the development of uncemented systems, the relatively high rate of loosening associated with uncemented stems spurred resurgence in the popularity of cemented stems.

Osteolysis tends to occur in certain areas. Zones 1, 7, 8, and 14 (all calcar bone) are commonly osteolytic, while zones 3 and 6, distally, may develop osteolysis following particle migration.

There are 2 approaches to the surgical treatment of osteolysis: removal of the lesion and removal of the cause of the lesion. First, the defect must be classified. The American Academy of Orthopaedic Surgeons (AAOS) classifies lesions as follows[4]:

  • segmental (cortical loss)

  • cavitary (contained lesion)

  • combined deficiencies

  • malalignment (rotational, angular)

  • femoral stenosis

  • femoral discontinuity

The AAOS has developed a treatment algorithm for osteolysis as well as several "pearls" for surgery

[5]

  • create a wide exposure to fully assess defects from inside and outside canal.

  • consider extended osteotomy.

  • curette osteolytic membrane.

  • reinforce defects with strut grafts; and

  • fill endosteal cavities with crushed allograft.

Choosing between a modular and a traditional prosthesis is also important. Advantages of modular components include:

  • independent proximal and distal sizing in diameter and length;

  • anteversion adjustment;

  • various degrees of porous coating; and

  • options for plasma spray coating.

Disadvantages of modular components include:

  • breakage at junction;

  • increased cost; and

  • theoretical fretting or corrosion.

In this case, the surgeon chose the Revision Hip ZMR system by Zimmer, a cementless titanium modular component with a plasma spray type of texturing that allows for independent proximal and distal sizing as well as multiple options for offset to increase stability. The stem has a lower modulus of elasticity, which leads to less stress shielding and less thigh pain.

Q. How does the age of the patient affect the choice between cemented and cementless stems?

A. Because younger patients are more active, they tend to do better with cementless stems, which support higher demand. Although hybrid replacements are the accepted standard of care for elderly patients, young and high-demand patients will expose the cement mantle to such high loads that it will fail early on, as in the case presented here.

Q. What do you believe is the real significance of debonding?

A. The debate over debonding may soon be moot. Initially, all stems were cemented, and a low percentage of them developed osteolysis. In fact, osteolysis was first termed "cement disease." Two schools of thought eventually developed. One urged eliminating cement altogether and only relying on cementless stems. The other faction advocated for improved stem-cement and cement-bone bonds.

The process was then termed aseptic loosening, which is actually a form of osteolysis. First, different cementing techniques were developed. Currently we're using third-generation techniques, which improve cement fixation to cancellous bone. Some authors advocate the use of textured surfaces, which actually bond to the cement. But if a roughened stem loosens, it becomes like sandpaper, causing rapid osteolysis by generating large amounts of cement particulate debris. As a result, there has been a movement back to smooth or polished stems that do not obtain as rigid a fixation to the cement.

Q. What are the real trade-offs between modular and nonmodular stems?

A. Nonmodular stems obtain reliable fixation distally in the femoral shaft. The downside is stress shielding, in which the femur doesn't see stress and the whole proximal bone melts away. Is that a clinical problem? Yes, because you occasionally see fractures of the greater trochanter that often cannot be repaired. Another problem is thigh pain, with a 10% to 25% incidence. That's where the idea of modular stem came from; you want contact proximally and distally. By using a titanium component, the modulus is decreased and thigh pain is less common. The ZMR system is about one and a half years old, and is designed to provide reliable proximal and distal fixation.

Q. Describe the surgery performed in this case.

A. The stem in this case was loose, with the cement mantle still attached to bone. Our options were to dig it out with ultrasonic tools or to do an osteotomy. We opted for an osteotomy and were able to get rid of all cement, even the interdigitated cement, which chisels may not have removed.

We also thought about our approach, given that this patient had suffered avascular necrosis (AVN) following synovectomy of the hip. How could we prevent that? European surgeons say we use the Bovie too much and that an anterior approach should, theoretically, preserve the blood supply. In Europe, where they perform a lot of open procedures in which they dislocate the hip and then put it back into place, they use sharp dissection, not the Bovie. But AVN doesn't usually follow pigmented villonodular synovitis. The approach through which I felt I could accomplish all goals quickly and efficiently was the extended trochanteric osteotomy.

Q. Finally, what about impaction grafting?

A. We reserve impaction grafting for patients whose osteolytic process has created such a large canal that no standard stems will fit in it. In this case, there was no need for it.

Made possible through an unrestricted educational grant from Zimmer.

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