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Thursday, March 16, 2000

Acetabulum may be enlarged after THR

Large hemispherical cups ("jumbo-cups") are commonly used in revision total hip re-placement (THR) to reconstruct the enlarged acetabulum, but this procedure is limited by the degree of coverage that can be obtained from the available bone stock, said researchers in a study in poster exhibit 58.

In cases where marked deformities or significant bony defects are present, more complex procedures must be considered, including the use of elongated shells and structural allografts, the researchers said. The anatomic study was performed to determine the size of the largest hemispherical shell that may be accommodated within the acetabulum without loss of bony coverage and to determine the change in the center of rotation of the hip joint caused by implanting an over-sized shell.

"Our results demonstrate that the average acetabulum may be enlarged by 6mm (11 percent) after primary THR without loss of adequate bony coverage," said Philip C. Noble, PhD, researcher. If the center of the primary cup has migrated by more than 9mm laterally or superiorly, adequate coverage cannot be obtained using an oversized hemispherical cup without excessive violation of the anterior or medial walls of the acetabulum. "In the average acetabulum, oversized cups can be used to fill bony de-fects measuring an average 8mm superiorly and 7mm posteriorly without the need for additional bone graft. However, in some cases, the reduced anteversion of the implant may increase the risk of dislocation."

Eight cadaveric hemi-pelves (acetabular diameter: 45 to 48mm) were concentrically reamed from 54mm to 61mm in 1mm increments. After each stage of reaming, a sur-rogate hemispherical cup was implanted in the acetabulum of the same diameter as each reamer. Reaming was terminated if the cup became grossly unstable or if adequate bony coverage was unobtainable without excessive violation of the anterior or medial walls. The cup was considered adequately covered if no more than 20 degress of its surface was exposed posteriorly and less than 10 degress was exposed superiorly.

Bone loss was considered excessive if the anterior wall did not remain intact or if the defect reamed through the medial wall was longer than one-third of the diameter of the socket. At each stage of reaming, the precise position and orientation of the surrogate cup was measured with respect to the bony pelvis us-ing a robotic 3-D digitizer (accuracy: 0.05mm). Using custom software, the researchers calculated the change in the position of the center of the acetabulum and the displacement of the surface of the shell. The inclination and the anteversion of the hemi-spherical cup were also determined.

After initial reaming, each speci-men accommodated a 54mm primary acetabular component. With sequential reaming, each acetabulum could be enlarged to 60-61mm without excessive loss of cov-erage or stability. The displacement of the cup center was highly variable, but averaged 3.51.8mm posteriorly (range: 1.4 to 6.2mm), 4.7 2.5mm superiorly (range: 0.8 mm to 8.6mm) and 0.93.5mm laterally (range: -3.3 to 8.4mm). This corresponded to an average dis-placement of the edge of the cup of 6.51.8 mm (4.4-9.2mm) posteriorly, 7.72.5mm (3.8-11.6mm) superiorly and 3.93.5mm laterally.

The inclination of the reamed acetabulum did not change significantly with sequential reaming. However, there was a dramatic reduction in anteversion of the implanted components compared to the initial (primary) cup. Although the average anteversion of the 61mm shell was only 4.0 3.9 degress less than the native acetabulum, the anteversion of individual specimens was highly variable, ranging from -20.1 to 12.9 degress.

The researchers are Philip C. Noble, PhD, director of orthopaedic research, Baylor College of Medicine, Houston, Texas; Stefano Landi, MD; Emir Kamaric, MS, Institute of Orthopaedic Research and Education, Houston, Texas; Francesco Fanton, MD; and Anthony Hedley, MD.

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Last modified 16/March/2000 by IS