As the symptoms of joint degeneration worsen, the classical surgical treatment option is joint arthroplasty. For joints such as hip and knees there are many options for patients. By contrast, vertebral disc degeneration classically would only have the option of conservative treatment and spinal fusion. Spinal fusion has its own negative side effects ranging from joint immobility to adjacent disc degeneration. More recently intervertebral disc replacement is available for those suffering degenerative disc disease. Intervertebral disc replacements are becoming a more common treatment option of severe disc degeneration. Like in peripheral joint replacements Intervertebral disc replacements have various component materials that they can be manufactured with. Currently multiple disc replacement are available such as the Dynesys, MC-6, and the Bryan disc to name a few. Commonly, there are three main materials used:Titanium plates that fixed in vertebral bodies of the spine, a polycarbonate urethane (PCU) fixed between the two titanium plates, and Ultra High Molecular Weight Polyethylene cord (UHMPE). Both the UHMWPE and PCU can produce wear debris and the particles can be associated with initiating an inflammatory reaction. Their presence in tissues may not be immediately apparent as both are clear polymers and difficult to see in standard histological preparations. However, when viewed with polarized light, both polymers are birefringent. Differentiating one from the other on the basis of polarized light appearance has not been studied. From previous work done at UCLA, and from a review of the orthopaedic literature, it has been established that UHMWPE in tissue sections has been successfully stained with Oil red O. The following question was postulated- was there a differential stain or Spectroscopy technique that could differentiate particulate PCU from UHMWPE. This would allow further analysis of the material breakdown that has been seen in vertebral disc implants in vivo, as well as improving the analysis of the UHMWPE and PCU tissue responses. The technique could be applicable to other implants using PCU biomaterials. The importance of this is to further understand the impact of vertebral disc implant failure.
A literature search was first performed toidentifying techniques that could allow us to differentiate PCU debris from UHMWPE. One article reported that Sudan black, a lipophilic stain, has been successfully used on PCUin vascular grafts that use PCU solely (Soldani, 2001). To test the staining potential of Sudan black for PCU from spinal device, macro debris was generated from vertebral implant parts made of UHMWPE and PCU. Particles were generated by grinding a PCU block and braided UHMWPE cord from a Cervical disc implant. These particles were analyzed by transmitted polarized light for characteristics. Staining was initially performed on these particles based on published techniques (Hoeksma, 1988).In addition, polarized light was used study morphological differences in PCU and UHMWPE on Hematoxylin and Eosin tissue sections. Routine paraffin-embedded histological serial sections from patients that have undergone spinal implant revision were de-paraffinized and stained as follows; 1. Sudan black (PCU), 2. ORO (UHMPE) and 3. Sudan black in combination with ORO for both polymers. Serial tissue sections were stained using ORO separately, and in combination with Sudan black to analyze dual stain compatibility. The use of both stains is to differentiate the UHMWPE and PCU debris in tissue sections that have undergone retrieval.
Histologically the bulk debris of PCU appeared Smooth and clear under transmitted light and uniformly birefringent under polarized light by contrast the UHMWPE cord also birefringent showed a characteristic ribbed pattern under polarized light. The PCU debris had a wide range of lengths and widths from 20 microns to >300 microns, UHMWPE on the other hand had a large range of length but a characteristic 30-micron average width. The ribbed pattern was also seen at an average interval of 28 microns. PCU and PE bulk debris were stained with Sudan Black and both polymers stained a Black/blue however the UHMPE could be de-stained with alcohol while PCU seemed to be permanently stained. These findings on the bulk indicated the possibility of staining microscopic PCU debris that were believed to be found in histological sections. Staining the microscopic sections with Sudan black only, the PCU stained a dark blue/black color which was similar to what was observed in the bulk debris findings. Following these findings with Sudan black next was confirming that ORO would stain UHMWPE as noted in orthopaedic literature. De-paraffinized spinal serial sections stained with ORO to confirm UHMWPE debris and stained red as reported in literature. Finally, both stains were used on histological sections from retrieved failed vertebral implants. Findings using both ORO and Sudan black on tissue section were consistent with what was seen in single stain procedures as the PCU stained dark blue/ black and the UHMWPE chord stained red.
Sudan black has been successfully shown to be suitable stain to use on PCU debris in both bulk material and more importantly clinically relevant debris produced by wear of the spinal disc implants. From our findings Sudan black staining protocol is a reasonable technique to be used as a differentiating stain in cases with PCU and UHMWPE. While this study was limited by the materials used we believe that it can be used in other devices that also use PCU as a component. In addition, the microscopic findings noted with the use of polarized light mentioned is also a technique to use in conjunction with staining to characterize histological findings. One of the most applicable parts of these results is that the staining protocol is cost effective as the materials used are relatively inexpensive and the procedure time is short.