Pseudo-Pedicle Heterotopic Ossification From Use of Recombinant Human Bone Morphogenetic Protein 2 (rhBMP-2) in Transforaminal Lumbar Interbody Fusion Cages
TAKE-HOME POINTS
- Use of rhBMP-2 in TLIF cages can result in HO out of the cage into the spinal canal.
- HO from rhBMP-2 in TLIF cages can result in a radiculopathy from compression or inflammatory reaction.
- HO out of the cage into the spinal canal resulting from use of rhBMP-2 in TLIF cages can be adequately diagnosed only with CT.
- HO can appear as a pedicle or pseudo-pedicle.
- Consider potential HO when using rhBMP-2 in TLIF cages.
RESULTS
All 38 selected patients had radiculopathy symptoms from HO out of the intervertebral space. The Table lists the patients’ overall characteristics. The left side had the most radiculopathy symptoms (31/38 patients), followed by the right side (5/38) and both sides (2/38). Radiculopathy symptoms began a mean (SD) of 3.8 (1.0) months (range, 2-6 months) after index surgery. The 38 patients had 4 characteristics in common:
Table. Transforaminal Lumbar Interbody Fusion With Recombinant Human Bone Morphogenetic Protein 2: Onset Time for Radiculopathy Symptoms, Surgery Level, Side of Pseudo-Pedicle Bone Formation, and Subsequent Complications
| Pt | Sympton Onset, mo | Surgery Level(s) | Side(s) | Complication(s) |
| 1 | 3 | L3-L5 (2) | Both | Radiculopathy, pseudo-pedicle, urine |
| 2 | 3 | L4-L5 (2) | R | Radiculopathy, pseudo-pedicle |
| 3 | 4 | L5-S1 (1) | R | Radiculopathy, pseudo-pedicle |
| 4 | 5 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 5 | 4 | L4-S1 (2) | L | Radiculopathy, pseudo-pedicle, subsidence |
| 6 | 5 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 7 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 8 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 9 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 10 | 2 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 11 | 2 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle, subsidence, neurologic |
| 12 | 6 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 13 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle, neurologic |
| 14 | 2 | L2-L3 (1) | R | Radiculopathy, pseudo-pedicle |
| 15 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 16 | 3 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
| 17 | 3 | L2-L3, L4-L5 (2) | L | Radiculopathy, pseudo-pedicle |
| 18 | 3 | L4-L5, L2-L3 (1) | L | Radiculopathy, pseudo-pedicle, nonunion |
| 19 | 4 | L4-L5 (1) | R | Radiculopathy, pseudo-pedicle |
| 20 | 5 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
| 21 | 5 | L5-S1 (1) | R | Radiculopathy, pseudo-pedicle |
| 22 | 3 | L3-L4, L5-S1 (2) | Both | Radiculopathy, pseudo-pedicle |
| 23 | 4 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
| 24 | 6 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 25 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 26 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle, urine, bowel |
| 27 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 28 | 4 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
| 29 | 6 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 30 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 31 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 32 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 33 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 34 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 35 | 4 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 36 | 3 | L5-S1 (1) | L | Radiculopathy, pseudo-pedicle |
| 37 | 4 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
| 38 | 4 | L4-L5 (1) | L | Radiculopathy, pseudo-pedicle |
1. Bone growing out of the annulotomy site for TLIF cage placement was present and in continuity with the disk space in 33 (87%) of the 38 cases. In the other 5 cases (13%), HO was present around the neural tissue, but not necessarily in continuity with the disk space. This bone appeared ectopic and not osteophytic and facet-related, as it formed a shell around either the nerve root or the thecal sac, contouring to the structure.
2. The common, novel finding on CT was a “pseudo-pedicle” (Figures 1A, 1B), which appeared as ectopic growth from the disk space—a solid piece of bone in the same direction as the anatomical pedicle. Confusing similarity to the anatomical pedicle is present on axial cuts and during surgery. The pseudo-pedicle varied in thickness and extent out of the disk space, but was always presented as a bar of bone arising from the annulotomy site. After arising from the disk space, the HO could disperse in any direction, further calcifying neural structures or the facet joints above or below. There was no apparent distinguishable repeating pattern, given the variable nature of arthritic facet changes, scoliotic deformities, size of annulotomies, amount of rhBMP used, and placement in cage and disk space or only in cage.
3. In 36 (95%) of the 38 cases, the initial interpretation of HO on magnetic resonance imaging (MRI) was of tissue other than bone, such as fibrous tissue, granulation tissue, recurrent disk herniation, or postoperative changes. However, this tissue was later determined to be bone from HO complications, which we confirmed with CT in all 38 cases. It is important to note that HO on MRI (Figures 2A, 2B) was initially interpreted by a radiologist as fibrous tissue, but same-level CT of the same case (Figures 3A, 3B) showed clear HO.
4. The radiculopathy symptoms caused by HO were independent of the amount of rhBMP-2 used in TLIF. Of the 38 patients, 19 had 1 rhBMP-2 sponge placed in the cage, 12 had a small kit sponge (1.05 mg), 5 had 1 sponge placed in the cage and 1 sponge placed directly in the disk space before cage placement (no notation of precise size or amount of rhBMP-2), and 2 had 1 sponge placed in the cage (no notation of rhBMP-2 amount). The data showed that HO can occur with even a small amount of rhBMP-2.
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