Computed tomography validating bony ingrowth into fibula strut allograft: a criterion for fusion

Abstract

Background context: Static and dynamic X-ray studies routinely determine whether or not fusion of a fibula strut allograft has occurred after multilevel anterior corpectomy with fusion (ACF) combined with posterior wiring and fusion (PWF). Purpose: Two-dimensional (2D) computed tomography (CT) studies were assessed for documentation of bony ingrowth into fresh frozen fibula strut allograft to constitute an additional sign of fusion. Study design/setting: This was a prospective, nonrandomized study, which was conducted at a university medical center. Patient sample: Eighteen patients with moderate to severe myelopathy undergoing circumferential cervical surgery for ossification of the posterior longitudinal ligament and spondylostenosis were evaluated. Outcome measures: Static and dynamic X-rays and 2D CT examinations were performed in 18 patients 3 and 6 months after circumferential cervical procedures. Methods: Fusion was assessed on static and dynamic X-rays, and 2D CT studies performed in 18 patients following average 2.9 level anterior corpectomy with fusion (ACF) with posterior wiring and fusion (PWF) (C2–T1) with halo application. Routine fusion criteria on static radiographs included the documentation of bony trabeculation and absence of bony lucency at the graft/vertebral end plate interface. Routine dynamic X-ray criteria of fusion mandated that less than 3.5 mm of translation, less than 20 degrees of angulation, and less than 1 mm of motion be observed between adjacent spinous processes. Here, a potential additional 2D CT criterion for fusion, progressive bony ingrowth into the central shaft of a fibula strut allograft, was investigated on 3 and 6 month postoperative 2D CT examinations using direct measurement of Hounsfield units. Telescoping (mm) was also differentiated from ingrowth into the bony shaft based on a comparison of immediate, 3 and 6 month postoperative CT studies. Results: Immediate postoperative baseline 2D CT studies revealed no bone within the central canal but an average of less than 1 mm of cephalad and 2.3 mm of caudad graft telescoping. Within 6 postoperative months, 2D CT studies demonstrated an average of 3.5 mm of superior and 4.6 mm of inferior bony ingrowth (confirmed by measuring 500 to 900 Hounsfield units) into the central fibula canal of 17 of 18 patients (94%). Seventeen had routine/dynamic X-ray and CT studies confirming fusion. Conclusions: Two-dimensional CT evidence of bony ingrowth into the central canal of fibula strut allografts after multilevel ACF/PWF provided an additional means of quantifying the extent of fusion.