[Analysis of the Design of the Ascension® PyroCarbon PIP Total Joint Proximal Component in Relation to the Proximal Phalanx Morphology].

BACKGROUND: There are many indications that long-term stability of the Ascension(®) pyrocarbon proximal interphalangeal (PIP) total joint is not attained by osseointegration but by appositional bone formation. Accordingly, good locking by cortical bone for sufficient primary stability is necessary in order to avoid prosthesis loosening before enough new bone has been formed. The size of the proximal component of the Ascension(®) PyroCarbon PIP total joint and thus cortical anchorage and primary stability depend on the morphology of the proximal finger phalanx and, especially, on the isthmus of the distal medullary canal.

OBJECTIVE: To clarify whether the medullary canal of the proximal finger phalanx can be enlarged using the Ascension(®) pyrocarbon PIP total joint instruments so that cortical contact of the proximal prosthesis component is guaranteed in the region of the isthmus in both sagittal and frontal planes.

MATERIAL UND METHODEN: Before insertion of the Ascension(®) pyrocarbon PIP total joint prosthesis, the isthmus widths of the proximal phalanx of 138 cadaver fingers were measured on radiographs by fluoroscopy in both the frontal and sagittal plane. The measured dimensions were compared to the dimensions of the inserted proximal component stems (manufacturer's instructions) after preparation of the medullary canal. The largest prosthesis possible was always used.

RESULTS: The mean isthmus width of the index fingers on PA view was 3.4 mm (2.4-4.2), of the middle fingers 3.3 mm (1.8-4.6), of the ring fingers 3.2 mm (2.2-4.7), and of the small fingers 2.9 mm (1.4-4.3). The mean isthmus width of the index fingers on lateral view was 2.6 mm (1.6-3.7), of the middle fingers 2.5 mm (1.8-3.8), of the ring fingers 2.3 mm (1.5-3.6), and of the small fingers 2.0 mm (1.4-2.8). There were statistically significant differences between the isthmus widths measured before insertion and the stem dimensions of the inserted proximal components. While the average width of the isthmus in the index, middle, ring, and little fingers was significantly larger than the stem width of the inserted proximal components, the average isthmus height was significantly smaller than the stem height of the inserted proximal components.

CONCLUSION: Ideal cortical contact between the isthmus of the proximal phalanx and the largest proximal component of the Ascension(®) pyrocarbon PIP total joint prosthesis that can be inserted, and thus good primary stability, is not possible due to a mismatch in the frontal plane between the morphology of the proximal phalanx and the prosthesis. But good contact can be achieved in the sagittal plane by broaching the medullary canal.