Complications after spinal fusion surgery are common, with implant loosening occurring in up to 50% of osteoporotic patients. Pedicle screw fixation strength reduces as a result of decreased trabecular bone density, whereas sublaminar wiring is less affected by these changes. Therefore, pedicle screw augmentation with radiopaque sublaminar wires (made with Dyneema Purity® Radiapque fibers, DSM Biomedical, Geleen, the Netherlands) may improve fixation strength. Furthermore, sublaminar tape could result in a gradual motion transition to distribute stress over multiple levels and thereby reduce implant loosening. The objective of this study is to test this hypothesis in a novel experimental setup in which a cantilever bending moment is applied to individual human vertebrae.

Thirty-eight human cadaver vertebrae were stratified into four different groups: ultra-high molecular weight polyethylene sublaminar tape (ST), pedicle screw (PS), metal sublaminar wire (SW) and pedicle screw reinforced with sublaminar tape (PS+ST). The vertebrae were individually embedded in resin, and a cantilever bending moment was applied bilaterally through the spinal rods using a universal material testing machine. This cantilever bending setup closely resembles the loading of fixators at transitional levels of spinal instrumentation.

The pull-out strength of the ST (3563 ± 476N) was not significantly different compared to PS, SW or PS+ST. The PS+ST group had a significantly higher pull-out strength (4522 ± 826N) compared to PS (2678 ± 292N) as well as SW (2931 ± 250N).

The higher failure strength of PS + ST compared to PS indicates that PS augmentation with ST may be an effective measure to reduce the incidence of screw pullout, even in osteoporotic vertebrae. Moreover, the lower stiffness of sublaminar fixation techniques and the absence of damage to the cortices in the ST group suggest that ST as a stand-alone fixation technique in adult spinal deformity surgery may also be clinically feasible and offer clinical benefits.

Focal knee resurfacing implants (FKRIs) are typically intended to treat focal cartilage defects in middle-aged patients. All currently available FKRIs are (partly) composed of metal, which potentially leads to degeneration of the opposing articulating cartilage and hampers follow-up using magnetic resonance imaging (MRI). The purpose of this study was to investigate the in vivo osseointegration process of a novel non-degradable thermoplastic polycarbonate-urethane (TPU) osteochondral implant. Bi-layered implants measuring 6 mm in diameter, with a double-curvature to match the approximate curvature of the goat medial femoral condyle were fabricated. TPU implants were composed of an articulating Bionate® II 80A top layer, and a Bionate® 75D bottom layer (DSM Biomedical, Geleen, the Netherlands) which is intended to osseointegrate. A biphasic calcium phosphate coating formulation, optimized during a prior in vitro study, was applied to half of the TPU implants, while the other half was left uncoated. Bi-layered metal implants (articulating cobalt-chromium top layer and titanium bottom layer) were used as positive control implants. Eight implants per group were implanted bilaterally in the medial femoral condyle of the stifle joints in 12 Dutch milk goats. 18F-sodium fluoride (18F-NaF) positron emission tomography-computed tomography (PET-CT) scanning was performed at 3 and 12 weeks postoperatively, and the corrected maximum standard uptake values (cSUVmax) was calculated to assess the peri-implant bone metabolism. After sacrifice 12 weeks postoperatively, bone histomorphometric analysis was performed to assess the bone-to-implant contact area (BIC). Student's T-test was used in case of normal distribution and the Mann-Whitney-U-test was used in case of abnormal distribution for comparison of BIC and cSUVmax. The BIC value of 10.27 ± 4.50% (mean ± SD) for the BCP-coated TPU implants was significantly (P=0.03) higher than the 4.50 ± 2.61% for the uncoated TPU implants. The uncoated TPU implants scored significantly (P=0.04) lower than the BIC of 12.81 ± 7.55% for the metal implants, whereas there was no significant difference between BCP-coated TPU implants and the metal implants (P=0.68). There was a strong correlation between the cSUVmax values and the BIC values at 12 weeks (Pearson's R=0.74, P=0.001). The cSUVmax values significantly decreased between 3 and 12 weeks for the metal implants (p=0.04). BCP-coated TPU implants followed a similar trend but did not reach statistical significance (p=0.07). cSUVmax in the uncoated TPU implants did not show a significant difference between the time-points (p=0.31). Osseointegration of BCP-coated TPU implants did not significantly differ from metal implants. 18F-NaF PET-CT is a feasible modality to assess osseointegration patterns and showed a similar trend between the BCP-coated and metal implants. Hence, an implant fully composed of TPU may avoid the typical metal-related drawbacks of currently available FKRIs. Long-term follow-up studies are advocated to address the effects of the implant to the opposing cartilage, and are therefore warranted.

INTRODUCTION

Adequate osseointegration of knee resurfacing implants for the treatment of focal cartilage defects is an important prerequisite for good clinical outcomes. Inadequate initial fixation and sustained micromotion may lead to osteolysis and ultimately implant failure. PET/CT with the bone seeking tracer 18F-sodium fluoride (18F-NaF) allows for localisation and quantification of abnormalities in bone metabolism. 18F-NaF PET/CT has been shown to correlate with loosening of implants in the hip and spine. Here, we asses osseointegration of the knee resurfacing implants using micro-computed tomography (µCT) and correlate µCT parameters to 18F-NaF uptake on PET/CT scans taken 3 and 12 weeks after surgery. We hypothesize that 18F-NaF uptake at 12 weeks and its relative decrease between 3 and 12 weeks correlates with osseointegration at 12 weeks postoperatively. Polymer implants with Young”s moduli approximately equal to- and below the Young's modulus of bone, with- and without surface modification were used in this study next to a control metal implant.

INTRODUCTION

Growth-guidance constructs are an alternative to growing rods for the surgical treatment of early onset scoliosis (EOS). In growth-guidance systems, free-sliding anchors preserve longitudinal spinal growth, thereby eliminating the need for surgical lengthening procedures. Non-segmental constructs containing ultra-high molecular weight polyethylene (UHMWPE) sublaminar wires have been proposed as an improvement to the traditional Luque trolley. In such a construct, UHMWPE sublaminar wires, secured by means of a knot, serve as sliding anchors at the proximal and distal ends of a construct, while pedicle screws at the apex prevent rod migration and enable curve derotation. Ideally, a construct with the optimal UHMWPE sublaminar wire density, offering the best balance between providing adequate spinal fixation and minimizing surgical exposure, is designed preoperatively for each individual patient. In a previous study, we developed a parametric finite element (FE) model that potentially enables preoperative patient-specific planning of this type of spinal surgery. The objective of this study is to investigate if this model can capture the decrease in range of motion (ROM) after spinal fixation as measured in an experimental study.

Summary Statement

Novel radiopaque UHMWPE sublaminar cables may be a promising alternative to gliding pedicle screws or titanium sublaminar cables within a growth-guidance system for the surgical treatment of early onset scoliosis.