We described the clinical characteristics of early and delayed symptomatic-RBF-SAP post-PTED and showed that intractable radicular leg pain was the most common symptom and was caused by ipsilateral or contralateral residual bone fragment, which could be removed through revision surgery.
Foraminoplasty in PTED
All patients with symptomatic RBF-SAP underwent foraminoplasty during PTED. Foraminoplasty involves widening the foramen by undercutting the ventral part of the SAP, with ablation of the foraminal ligament, using bone trephines or an endoscopic drill and side-firing lasers, to visualize the anterior epidural space and its contents. Foraminoplasty was performed to avoid damaging the exiting nerve during endoscopic cannula insertion into the neural canal through the intervertebral foramen. Henmi et al. evaluated the intervertebral foramen pre- and postoperatively using CT and found that the mean foraminal area increased significantly from 58.6 to 88.4 mm2. However, bone destruction increases the possibility of residual bone fragment. Foraminoplasty is performed blindly and depends greatly on the surgeon’s experience. Some bone fragment may not be removed after the SAP is cut. When the site of foraminoplasty is not ideal, adjusting the direction of the working cannula is often necessary to improve decompression. In elderly patients, care should be taken to reduce the risk of residual bone fragment due to osteoporosis. Foraminoplasty demands on the surgeon’s technical skill and experience, and remains challenging even for skilled surgeons.
Types of residual bone fragment according to time, location, and shape
In the delayed cases, symptoms appeared following a period of improvement after they had been discharge. Two of the seven patients complained of radicular leg pain 1 week after discharge and another complained 1 month after being discharged. A residual bone fragment was diagnosed using CT in both patients. The reason for delayed symptoms may be that the location of the bone fragment changed with activity, and symptoms appeared when the dural sac or exiting nerve root was compressed by the displaced bone fragment. Irrespective of whether conservative treatment is effective, surgical removal of bone fragment is recommended. The bone fragment is completely separated from the ligament; therefore, it can move anywhere in the spinal canal during conservative treatment, and symptoms may appear gradually, even on the opposite side. Moreover, it can cause injury to the nerve root and dural sac.
In contrast, in early cases, symptoms appeared soon after the surgery. Within hours after PTED, patients could not sit or walk because of intractable radicular pain, which was similar to mechanical compression of the nerve root and dural sac. In osteotomy surgery, neurological deficits can be caused by inadvertent placement of implants or compromise of the blood supply to the cord. However, no implant was used, and the dura did not move during PTED. It is difficult for surgeons to determine whether symptoms are caused by residual bone fragment. In all seven cases, the rare complication was recognized using CT or secondary intraoperative exploration; no fragment was found during the initial surgery. In PTED, patients are awake and under local anesthesia and can provide instant feedback to surgeons. One of our patients complained of leg pain when the surgeon adjusted the working cannula intraoperatively, but the surgeon considered this to be a normal phenomenon during surgery. In such cases, a bone fragment may be found by exploration, which can prevent rare complications and secondary surgery. Alternatively, a CT scan can be performed during surgery (if conditions permit). For open surgery, residual bone fragment can reportedly cause neurological deficits, including nerve root, cauda equina, and spinal cord deficits. In our study, the incidence of symptomatic-RBF-SAP was 1.2%; however, although the rate was low, its effect on patients cannot be ignored. Additionally, asymptomatic RBF-SAP was encountered, but the rate was low. In our study, only nerve root deficit occurred, possibly due to the anatomy of the intervertebral foramen and exiting nerve root. The residual bone fragment of the SAP after foraminoplasty becomes a foreign body in the foramen that can be pushed into the spinal canal by inserting a working cannula after foraminoplasty. Due to the limited space in the spinal canal, symptoms of nerve root deficit can easily occur.
In the early cases, two patients experienced pain on the contralateral side. Typically, people will consider recurrence at the same location or other levels first, which may be due to activity or an undiscovered herniated intervertebral disc. In one of our patients, a mixed signal in the spinal canal was detected using MRI (Fig. 3), although CT was not performed after the operation, MRI found only a hematoma. Although epidural hematoma is the most common cause of postoperative neurological deficits , a residual bone fragment was found during revision surgery in this patient. The bone fragment may have resulted from foraminoplasty and could have been pushed to the opposite side and moved across the midline during cannula placement. Interestingly, no contralateral cases occurred in the delayed type. Contralateral bone fragment move from side to side over time; thus, ipsilateral cases are expected to occur early.
The two largest bone fragment were in contralateral cases. These may have been easily pushed to the opposite side while manipulating the cannula, as small bone fragment may have been pushed around the working cannula. Jacob et al. noted that neurological deficits were always unilateral, were never proximal to, and usually did not correspond to the level of the osteotomy. This was thought to be due to a combination of subluxation, residual dorsal impingement, and dural buckling. Although the residual bone fragment was also unilateral in PTED, the deficit was at the same level. None of the patients had more than one residual bone fragment.
Residual bone fragment may also cause secondary complications unrelated to early or delayed types. Although there have been no reports of secondary complications in open surgery, this complication is related to the shape of the bone fragment. The sharp edge of the fragment can injure nerve roots and the dural sac. If the dural sac is punctured, CSF leakage occurs, and this increases the risk of infection. It may also cause neural entrapment syndrome when the nerve root is compressed by a dural tear. Two patients (28.6%) had CSF leakage, and one patient (14.3%) had nerve injury. Secondary complications are associated with the risk of recurrence if the fragment is not removed.
Revision strategy of residual bone fragment
Some studies have reported new neurological deficits caused by residual bone fragment after open spinal surgery[10, 19–21]. Some patients in these studies responded to conservative treatment, while others required revision surgery. With laminectomy, the spinal canal volume increases, and the probability of successful conservative treatment is increased. However, our PTED patients did not have changes in spinal canal stenosis; thus, conservative treatment was less useful. One patient with low back pain refused surgery because of fear, while the others underwent various types of revision surgery. The VAS score decreased in all patients, irrespective of the revision surgery type.