Histological remodelling of demineralised bone matrix allograft in posterolateral fusion of the spine – an ex vivo study

The current gold standard in thoraco-lumbar fusion surgery of the spine is posterolateral arthrodesis using autologous bone graft from the iliac crest in addition to instrumentation [1, 2]. Although autologous bone grafts provide ideal biological properties, graft harvesting may cause severe donor site morbidity [3–8]. Therefore, spine surgeons increasingly tend to use allogenic bone graft as osteoconductive and osteoinductive carriers enhancing solid fusion without disadvantages of graft harvesting. Demineralised bone matrix (DBM) has shown to be a reliable alternative in terms of its fusion capacities in several animal models as well as in clinical investigations in humans [9–17]. While histological performance of DBM was extensively investigated in animal fusion models [9–14], only few ex vivo studies were reported proving efficacy of this generally very expensive material. These studies dealt either with dental surgery procedures or idiopathic scoliosis in a child, whereas no ex vivo investigation was reported focusing on adult spine surgery [18, 19]. To our knowledge, the present study is the first report of histological performance of DBM in a consecutive series of patients who underwent posterolateral fusion of the spine. The goal of our investigation therefore was to clarify whether DBM incorporates to the surrounding bone or not. The hypothesis was set up that transformation of DBM is related to its time in situ (“dwell time”).

Nine patients (6 male and 3 female, 30 to 64 years old) that were scheduled for elective removal of dorsal instrumentation hardware over a 12-month time period were included in our study. Indications for hardware removal was subjectively disturbing hardware and the explicit patients wish to remove the implants in 8 out of nine patients. In one patient aseptic implant loosening was diagnosed. Osseous fusion was confirmed radiologically in all cases. For index operation 10 cc of DBX Mix® (SYNTHES, Oberdorf, Switzerland), a demineralised bone matrix substitute embedded in sodium hyaluronate mixed with cortical allograft bone chips, was used. The fusion bed was prepared using a high speed drill before DBX Mix® was added. No additional autologous bone graft harvesting of the iliac crest was conducted in all cases. Except of one female patient who presented pathological osteoporotic fracture with consecutive stenosis of the spinal canal, all other patients were initially operated for unstable traumatic vertebral fractures and healthy otherwise.

The time period between index operation and hardware removal and osseous biopsies respectively, was between 6 and 18 month (mean 10 month). No intraoperative and no postoperative complication including infection or extensive hematoma formation occurred in any patient. At the time of hardware removal, intraoperative testing showed solid fusion of all bridged segments in all cases and DBX Mix® seemed to be remodelled and integrated within the fusion mass.

Qualitative and semi-quantitative, histological analysis showed time dependent partial remodelling in some samples and almost complete remodelling of the DBM into new bone in other cases. Detailed assessment showed not only reorganization of demineralised matrix but also of the autologous and allogenic bone chips generated through preparing the graft bed during initial surgery. While samples of patients with shorter “dwell time” of DBM in situ showed less new bone formation and more DBM and chips residuals, samples with longer stays showed almost complete substitution of DBM with a lot of new bone mineralization and progressive remodelling of cortical chips (see Figures 1 and 2 and Table 1). Figure 3 shows the “creeping substitution” [22] in a case of 6 month DBM in situ. Precipitation of new osteoid and extensive matrix calcification is shown in Figure 4. On the thin slices different phases of the reorganizational process could prove osteoinductive and osteoconductive capacities of the DBM. For case related analysis see also Table 1.

Since DBM for clinical use is very expensive compared to autologous bone grafting, effectiveness of such products is mandatory. While DBM gel as an autograft extender showed improved fusion in dogs [7], heterologous DBM provided equal fusion capacity as autologous bone graft in rats [8]. Comparable results were also obtained when DBM was used as partial substitute of autologous bone graft in a postero-lateral fusion model in rabbits [9]. In another rabbit model, DBM showed osteoinductive ability, while allogenic deep-frozen cortical bone did not [10]. Radiographic investigations in humans showed equal fusion rates of postero-lateral arthrodesis performed in patients with local autologous bone graft augmented with DBM and in those with autologous bone graft from the iliac crest alone [13]. Radiographic progression of the fusion with time was also shown when DBM was used as bone graft extender to decrease the need of autologous bone graft [14]. Intra-individual comparison of DBM augmented autograft at one side and autograft without adjunct at the contralateral side in patients who underwent postero-lateral fusion showed equal results in radiographic assessment [15]. An ex vivo histological analysis of different types of DBM products used for sinus lift procedures in oral surgery showed superiority of DBX® in terms of new bone formation and low residual demineralised matrix compared to other products [16]. In a recently published case report of a 7-year old male child suffering from idiopathic scoliosis who underwent posterior spinal fusion, DBM was used as sole graft source. Eleven month later on the basis of a routine surgical exploration, osseous biopsies were taken from the fusion site and histological analysis was performed. In this case, investigators found only mature bone with no residual DBM graft and concluded therefore DBM to be fully incorporated in the fusion mass [17].

Our results tend to support these overall findings of reliable remodelling of DBM to new bone. The histological analysis showed significant new bone formation and decreasing residual DBM material in all of our patients depending on the time span DBM was in situ. Additionally, cortical bone chips converted as well in the same manner. The verifiable process of “creeping substitution” allowed confirming active incorporation and reorganisation of the demineralised matrix.

Further analysis should be made to determine whether additional cortical bone chips are necessary to enhance new bone formation or if DBM putty alone provides best performance in postero-lateral fusion of the spine. Comparative study designs will help to analyse the bone remodelling patterns of different treatment courses such as the use of DBM versus autologous bone grafting or spontaneous fusion based on posttraumatic hematoma.

Limitations of our study are the small number of patients included in the collective and the lack of quantitative morphometric analysis based on standardized bone samples, which of course was not applicable in the clinical setting. Furthermore the results of our investigation are not applicable to postero-lateral fusion in general since our study population is relatively youg (mean age 44 years) and the reason for a fusion was trauma in eight of nine cases. Additionally there may be a bias with regard to the harvesting site of the bony samples, which were all taken around the rods during implant removal and therefore may not be representative for all fusion areas.

DBX Mix® shows reliable remodelling in histological analysis and therefore seems to be an effective in postero-lateral fusion of the spine in the presented study population. Furthermore, DBM substitution increases over time.