Critical Review by kbQ7NR

VIEWS: 6 PAGES: 5

									                                                                        Nicholas A. Ramey
                                                                         February 23, 2001
Critical Review: “Treatment of Pelvic Osteolysis Associated with a Stable
Acetabular Component”

Statement of the Problem and Proposed Solutions:
Osteolysis is a general term for the degeneration of bone tissue. This discussion
examines pelvic osteolysis as a result of total hip arthroplasty, although it can result from
disease or infection. Specific mechanisms for the onset of osteolysis from THA are
under study, but research supports that acetabular component design is a central concern.
Short of reengineering the THA paradigm or of conducting an exhaustive biomaterials
study, proposed solutions for the prevention and treatment of osteolysis include: pre-
operative radiation sterilization, high-energy radiation ablation, and meticulous
debridement.

Background
Acetabular Componentry
The acetabular component of a total hip replacement is a mechanically important fixture.
Because this piece of mechanical hardware lies within the body, its design is chemically
important as well.

First, consider the bone-implant face of the component. Intuition tells developers that
mechanical stability can be gained through direct fixation with screws or at least a
maximal bone-implant contact. Likewise, many components are designed with through-
holes for bone screws; and others with a special porous texture, in conjunction with a pre-
reamed acetabulum of hemispherical shape. The hope is that the porous textures will
provide a sufficiently rough interface for bone integration. Still, other surgeons prefer to
use cement in fixing the component.

On the opposite face of the acetabular component, femoral component contact becomes
an issue. In this case, developers conceived of at least three primary interfaces, metal-
metal, ceramic-ceramic, and metal-plastic. The key topic to address with this interface is
not fixation, but wear. Jacobs found that a metal-metal interface introduced, among other
statistics, a 35-fold elevation of metal (chromium) in urine above other interfaces. To be
sure, the majority of recent studies reveal that both metal-plastic and ceramic-ceramic
interfaces introduce bone-degenerating material to the pelvis. This discussion in
particular addresses the polyethylene-metal shell interface, and how wear of this polymer
leads to osteolysis.

Osteolysis
Numerous studies show that acetabular osteolysis is very often asymptomatic, meaning
that the patient presents no visible symptoms. There proves to be no one cause of
osteolysis. This discussion examines osteolysis as a result of particulate penetration from
wear debris of total hip replacement components. Dr. John C. Clohisy of Washington
University studies the molecular effects of polymethylmethacrylate on the activation of
transcription factor (NF-B). This activation is thought to be responsible for
osteoclastogenesis, the major component of bone resorption or osteolysis. Osteolysis of
the acetabular region usually takes one of two paths.
Often in cemented components, a ten-year radiographic study will reveal a radiolucency
bordering the implant surface. In such cases of a linearly progressive lesion, the
component’s stability is jeopardized and its orientation has changed. This position
change in turn stresses the femoral component and introduces additional interfacial wear.

In cementless components, the osteolytic lesion is often diagnosed expansile, where
progression is perpendicular to the cortex, toward the bone center. This type of invasion
weakens the bone structure, and may lead to fracture, or destruction of intra-bone
vasculature.

Experts’ Approaches
“Treatment of Pelvic Osteolysis Associated with a Stable Acetabular Component
Inserted without Cement as Part of a Total Hip Replacement.” By William J. Maloney,
M.D..
General Theory
The authors of this clinical study examined stable acetabular implants whose pelvises
contain osteolytic lesions. Recognizing that frequency of osteolysis increases with
duration of follow-up time, their targeted sample had a minimum of three-years follow-
up time. Initial intuition told the authors that the best treatment for pelvic osteolysis
required updating the entire acetabular component. Early studies revealed that such
operations often lead to destruction of stabilizing bone pods and ingrowths. This
destruction could lead to further damage of the acetabulum, resulting in faulty component
integration and poor pelvic continuity. They present various alternative treatment
techniques for lesions surrounding different types of acetabular components. They
believed that a two-year minimum follow-up period provided sufficient results to
conclude positively on their treatments.

Methods
The experiment was a retrospective analysis of revisited-THA records. Of the 162
records reviewed for patients with revision THA between 1991 and 1994, thirty-five were
examined carefully because they were treated for pelvic osteolysis. These thirty-five
were approached with the following perspective:
       Retain a stable acetabular component associated with pelvic osteolysis if
           a. the component was designed for polyethylene liner replacement and a
              sufficiently thick liner was still available.
           b. the locking mechanism for the polyethylene liner still functioned correctly
              in securing the liner against the metal shell.
           c. the metal shell had not been damaged by the femoral component.
           d. the acetabular component was at a position promoting stable
              reconstruction in view of dislocation.
       Remove the acetabular component if
           a. a radiolucent line at the interface could be detected.
           b. the component had migrated (migration assessed using Massin’s method,
              examining both horizontal and vertical displacement from the teardrop).
In characterizing the lesion sizes, the authors measured its longest diameter and the
diameter perpendicular to the first. Such measurements were done radiographically, and
an example resides on page 1629, Fig.1 of the article. At the time of operation, the
stability of the femoral component was manually assessed (and dealt with accordingly).
Upon exposure of the acetabular component, hypertrophied or enlarged synovial tissue
was removed, as was the liner of the component. The following steps were taken for
assessment and treatment:
         1. competence of the liner locking mechanism was verified.
         2. stability of metal shell was verified using manual pressure.
         3. granulomatous tissue of the osteolytic lesions was removed. Access was
            gained either around the periphery of the component or through holes of the
            component.
         4. a new polyethylene liner was inserted, and the femoral head size was
            decreased to accomidate the thicker liner.

The authors used the following criteria for identifying a successful resolution of the
osteolytic defect:
       1. loss of a definite border between the osteolytic defect and bone.
       2. return of bone density to that of surrounding bone.
       3. development of networked pattern of connective tissue in the region
            excavated.
       4. Note: inability to remove entire defect “was not considered a contrindication
            to this procedure (Maloney 1630).

Results and Conclusions
Of the thirty-five patients, forty-six lesions were identified in regions including the ilium,
pubis, and ischium. The lesions’edges were well defined, and were expansile, so they did
not track the bone-implant interface. The following results were found upon recent
radiographic investigation:
        1. The components were still stable.
        2. No new lesions were visible.
        3. Borders of radiolucencies and bone became less defined.
        4. Densities became normal.
        5. Some of those whose cavities were not bone-graphed experienced some
            regression, others’ resolved fully (similar results were found with those given
            bone graphs.
Increased rate of polyethylene wear and associated prevalence of osteolysis resulted from
early porous-textured acetabuilar components. As noted, patients with osteolysis have no
associated pain. This identifies the importance of regular post-operative radiographic
check-ups. When the patient presents with painwith osteolytic lesioning, other causes
should be examined, implying that osteolysis may be involved in a fracture or other tissue
disruption.

It was concluded that a sustained particle load is an important varible leading to
osteolysis. This led the authors to believe that replacement of the old liner for a thicker
new one with less wear was more effective in osteolysis retardation than full evacuation
of the lesion. This belief was supported by the fact that none of the lesions grew after the
revision. Because the time frame of the study was somewhat limited, and because the
nature of osteolysis is still under study, the optimal surgical treatment is still unknown.
The authors determined that given the post-revision surgery results, their method of
lesion debridement was a practical alternative to complete replacement of the acetabular
component. It was also suggested that if a component is unstable, it should be revised. If
the locking mechanism is incompetent, either the component should be revised, or the
liner cemeted to the metal shell. Finally, thickness of the liner should be assessed, and
the surgeon should consider factors like activity level, overall patient medical condition,
life expectancy, etc. in deciding whether to revise the component or simply replace the
liner.

Personal Assessment
My approach to assessing the study is systematic. I view each step of the experiment –
including research topic, background, methods, results, and conclusion – from a
biomedical perspective. The “Treatment of Pelvic Osteolysis Associated with a Stable
Acetabular Component Inserted without Cement as Part of a Total Hip Replacement”
study was of particular interest to my research, which concerns treating pelvic osteolysis.

The study provides fair background information regarding the clinical scenario
surrounding this form of osteolysis, the possible precipitates, and the radiographic
analysis and verification of possible manifestations. This background does not address
other methods for removal of the pelvic osteolysis, perhaps because the problem is
relatively young and under-researched.

The methods of the study are not vigorously experimental. In fact, they rely on a small
sample of records on revised hips. These records were no more than six to seven years
old at the time of the study. The choice of records may have been limited given that the
researchers were interested in cement-less acetabular components. In this respect, they
upheld good experimental form, attempting to examine only one variable at a time.
Nevertheless, the short follow-up period and small sample size may have limited the
results and associated conclusions. Of course, it is understood that these researchers are
among the first to address osteolysis in stable, cementless components, thus their choice
for such a retrospective analysis may be appropriate for preliminary purposes.

The results of their study were presented in a straight-forward manner, although a simple
chart would have nicely accommodated the medical garb. Also, the language was
somewhat vague in the report of some of the results, and context clues were not
elucidating. I believe that because the conclusions were drawn on such a small sample
size, they may not be fully satisfying. The conclusion portion of the paper presented
some redundancies. However, the authors’ assertion that osteolysis resulted mostly from
sustained particle load was interesting, and their approach to resolution complimented
this belief. Overall, the authors suggested seemingly sound methods for approaching
pelvic osteolysis in revision THA that are applicable and important for consideration by
any surgeon.
Next Step
It is accepted that Harris’s opinion on the nature, history, and future of the total hip
replacement is the most useful in attempting to advance the paradigm. However, given
that the researchers are not mechanical or material science engineers, nor are they all
medical doctors, the most realistic next step for this study is the application of computer-
integrated surgery techniques for reliable assistance in excavating or debriding osteolytic
processes. This incorporates known techniques for image registration and robotic control
in accessing the cortex minimally invasively. Given that osteolysis is becoming more
frequent as implant age increases, the any solution has clear clinical advantages.
Unfortunately, because osteolysis is often asymptomatic, the patient may not feel that a
revision is necessary, or may in fact be weakened by another invasive surgery. This
team’s proposed solution addresses this issue directly. Realistically, however, a true
longitudinal study is necessary to weigh the full advantages of designing a computer-
integrated surgical system for this application.

Maloney, W.J. M.D. “Treatment of Pelvic Osteolysis Associated with a Stable
Acetabular Component Inserted without Cement as Part of a Total Hip Replacement.” J.
Bone and Joint Surg. 79-A, November 1997, pp. 1628-1634.

Harris-W-H. Department of Orthopaedic Surgery, Massachusetts General
Hospital,
Clin-Orthop m. 1992 Jan. (274). P 6-11.


Jacobs JJ, Skipor AK, Doorn PF, Campbell P, Schmalzried TP, Black J, Amstutz HC:
Cobalt and chromium concentrations in patients with metal on metal total hip
replacements. Clin Orthop 1996 Aug;(329 Suppl):S256-63

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