How Useful is MRI in Evaluating ALTR ?

Abstract

Biologic failures of hip replacement have emerged as an increasing threat to the longevity of the prosthesis. While wear of modern day bearings has been greatly reduced with the advent of cross linked polyethylene, local reaction to metal particles either from the bearing itself or to any of the modular tapers appears to be on the rise. Monitoring of these reactions by the use of plain radiographs or serum markers appears to be insufficient to gauge the gravity of the response.

Over the past decade, the use of magnetic resonance imaging (MR) techniques has emerged as the superior noninvasive instrument to assess the extent of soft tissue reaction around hip implants. The use of MR imaging around implants was initially challenging due to the presence of relatively ferrous metals especially cobalt which causes local distortion of the magnetic fields. Novel changes in pulse sequencing have greatly improved the sensitivity and specificity of MR imaging so that at this time, MR is the most predictive diagnostic tool in evaluating the extent of tissue destruction. We feel strongly that modern MR imaging techniques are the most important tool in the workup of the patient suspected of having an adverse tissue reaction after hip arthroplasty.

INTRODUCTION

Failure in total hip arthroplasty remains a reality. Bozic et al¹ reported that using the nationwide inpatient sample database, instability, mechanical loosening, infection and periprosthetic fracture were the leading causes for revision surgery. This analysis however, represents cases that were performed in the 1980’s and 1990’s. At that time, many of these failures could be attributable to osteolysis and the consequences of polyethylene wear.

The new millennium brought a number of changes to hip arthroplasty including the use of highly cross linked polyethylenes but also the era of metal on metal hip arthroplasty as well as modular hip implants. In addition, there were some modifications to the shape and design of the femoral trunnion. While all these changes were hoped to be improvements in implant performance, some have not withstood the test of time. We recently reported our review of more “modern day” hip failure. Our results have indicated that almost 30% of the failures of hip arthroplasty were due to biologic driven events from either metal-on-metal bearings or more problematic, modular connection problems leading to adverse tissue reaction².

One of the first indications of this new era of problems were the reports of the metal on metal hip experience. The new generation of MOM bearings were either in the form of large diameter head total hip replacement or hip resurfacing. Due to enhanced stability and perceived better functional results, adoption of MOM usage reached almost 35% by 2006³. Within a few years, published reports of early failures and adverse reaction to metal debris as well as a new phenomena of pseudotumor began to appear^4,5,6. Unlike reactions to polymeric debris which often resulted in osteolysis, these reactions were associated with peri-articular soft tissue destruction including the hip capsule as well as muscle-tendon units such as the abductors resulting in massive necrosis. While the presumed origin of the metal debris was the articular surface, some investigators noted that not all adverse reactions were observed in highly worn bearings⁷. Implying articular wear as the main driver of this adverse reaction lead to the recommendation of monitoring of serum ions as a surrogate for potential risk. Unfortunately, absolute level of serum ion concentration which might indicate a patient at risk for adverse reaction is unclear⁸. In addition, individuals may have elevated serum ion levels but not mount an inflammatory reaction to the particles. While significantly elevated metal ions can be associated with systemic manifestations⁹, the more common and problematic issue is the soft tissue damage occurring as a result of this aggressive biologic response. Unfortunately, plain radiographs as well as computerized tomography are not particularly useful in assessing the degree of damage. There was clearly a need to develop more novel ways to evaluate the biologic consequences around the periprosthetic space.

THE EMERGENCE OF MAGNETIC RESONANCE IMAGING AROUND ARTHROPLASTY

The biologic consequences of wear and metallic implants have been a concern for decades. Most these concerns focused upon the macrophage driven resorptive cascade leading to osteolysis. Unfortunately, it is well known that plain radiographs often underestimate the extent and magnitude of bone loss¹⁰. Computerized tomography appeared to be a very predictive diagnostic tool in assessing osseous resorption but is associated with large doses of ionizing radiation and may not be appropriate for continued surveillance of these lesions. It was these concerns that lead to the development of magnetic resonance as a newer method of evaluation. MRI has the advantages of using non-ionizing radiation techniques, with tomographic image acquisition and a superior ability to assess soft tissue content. “Traditional MRI” techniques are limited by susceptibility artifact due to the presence of ferrous materials (eg iron, cobalt), which can cause distortion of the local magnetic fields often rendering the images nondiagnostic. Using novel changes in the pulse sequencing of commercially available MR software such as three-dimensional multispectral imaging (3D MSI), the ability to demonstrate synovial characteristics, soft tissue envelope integrity as well as detection and extent of osteolysis was demonstrated^11,12 (fig 1–2).

Figure 1.

“traditional” fast spin echo MR technique with poor visualization around the implant.

Figure 2.

Pulse sequence changes resulting in improved visualization of periarticular joint space.

With the adoption of highly crossed linked polyethylene as the primary bearing in total hip arthroplasty, there has been a significant decrease in the incidence of polyethylene wear and osteolysis. However, other biologic associated tissue responses have been on the rise including patients having had prior metal-on-metal hip replacement, MOM hip resurfacing and the more recent described phenomena of trunnionosis¹⁴. The resulting adverse local tissue reaction (ALTR) observed in these patients is unfortunately often associated with extensive soft tissue destruction leading to significant disability making the outcomes of revision surgery unpredictable. It is therefore crucial to make the correct diagnosis promptly and consider more expedient intervention.

UTILITY OF MRI IN EVALUATING ALTR

Considering which patients are at risk for ALTR can be sub-categorized into patients with known “at-risk” implants or those with apparent low risk implants. “At-risk” implants include recalled implants such as some of the metal-on-metal hip replacements as well as modular neck prostheses. These patients may have few if any symptoms but need to be monitored carefully. While it is reasonable to evaluate these patients with screening metal ion levels, as mentioned previously these can be misleading. The patient in Figure 3 was approximately 8 years after a metal-on-metal THR with a complaint of mild “bursitis”. MRI was performed using metal artifact reduction technique¹¹ which demonstrated a large fluid cystic mass with marked synovial thickening which correlated with the intraoperative findings of necrosis (Fig 4,5). Revision of this MOM THR required acetabular shell removal and conversion to a ceramic on polyethylene bearing with elimination of symptoms.

Figure 3.

AP radiograph of a metal on metal THR

Figure 4.

MRI of patient with MOM THR demonstrating a large cystic mass overlying the trochanter

Figure 5.

Intraoperative photo demonstrating large cystic mass seen on MRI.

Patients with extensive symptoms require thorough workup including 3-D imaging using MRI. Progressive pain with worsening limp in a previously well functioning hip replacement is clearly a worrisome finding. MR imaging in this case of a prior MOM total hip demonstrated extensive capsular thickening with disruption of the hip abductors (Fig 6). Intraoperative findings included massive necrosis of the hip abductors and destruction of the posterior capsule (Fig7). Patients with this degree of soft tissue damage are at increased risk for instability after revision and unfortunately overall function is often compromised.

Fig 6.

MRI of patient with recalled THR and progressive limp. Note synovial thickening and signal change in the abductors

Figure 7.

Intraoperative photo of patient with necrotic abductors confirming findings on MRI.

Over the past decade, there have been an alarming number of reports of ALTR occurring in what was thought to be a “low risk” implant system: a metal on polyethylene bearing^14,15. Patient symptoms in this cohort can range from the very vague as new onset lateral hip pain to the more troubling situations of late onset instability. Plain radiographs are often of little value in that there is often osteolysis. It is only through cross sectional imaging can the diagnosis of adverse local tissue reaction be made. The apparent origin of this phenomena appears to be related to the complex relationship between the femoral head and trunnion. The specific features that initiate this process is not completely understood but factors ranging from taper design, taper length as well as head diameter have been studied^16,17.

Any patient presenting with new onset pain or symptoms such as limp or late dislocation need a complete evaluation. As with any problematic joint replacement, workup should include a thorough history, clinical exam including observation of gait and assessment of motor strength. Laboratory analysis should include screening labs for infection including complete blood count, erythrocyte sedimentation rate and C-reactive protein. Metal ion levels appear to have some value as part of the complete workup but should be interpreted accordingly. It is now routine protocol to obtain advanced imaging in the form of metal suppression MRI. In the presence of cobalt chromium components, 3DMSI MRI is essential to visualize the trunnion. Features that are of particular importance on the MRI include the presence of any solid or cystic masses or deposits of low signal intensity metal debris in the surrounding soft tissue. Perhaps most noteworthy is the observation of thickening and expansion of the synovium which is a highly predictive feature of the presence and severity of ALTR¹⁸.

A common scenario for ALTR observed in patients with a metal-on-poly bearing is described as follows. A 70 year old male who is 2 years after an uncomplicated THR who now complains of vague lateral hip discomfort and slight limp. His plain radiographs were unremarkable (Fig 8) and serum studies including ERS, CRP and metal levels were within normal standards. After failing a course of physiotherapy, he presented to our clinic for re-evaluation. At that time, he is sent for an MRI (Fig 9) which demonstrates synovial thickening and expansion consistent with adverse local tissue reaction. Findings at the time of revision reveal capsular thickening with extensive necrotic tissue (Fig 10). Revision to a ceramic head employing a titanium tapered sleeve was performed without incident and the patient had resolution of his symptoms.

Figure 8.

Patient with new onset lateral hip pain and limp 2 years after metal-on-polyethylene THR.

Figure 9.

MRI demonstrating marked synovial thickening (arrows) consistent with the development of ALTR.

Figure 10.

Intraoperative photo confirming the presence of thickened, necrotic synovium and capsule surrounding the femoral head.

SUMMARY

Biologic failures in total hip arthroplasty may be on the rise. Unfortunately, serum biomarkers do not yet have the sensitivity or specificity to predict when a patient is developing an adverse local tissue reaction. The use of three dimensional imaging in the form of MRI utilizing modern techniques to reduce the impact of artifact provides the clinician with a real time assessment of what is occurring at the prosthesis. At this time, MRI can tell us which patients are developing ALTR and can provide guidance as to when it may be appropriate to intervene.