Utilizing the ComposiTCP Screw for Interference Screw Fixation

An ideal anchor or interference screw material needs to provide strong mechanical fixation, with the ability to completely degrade over time and become replaced by bone. Initially anchors/interference screws were made of metal, followed by polymer-based implants, and most recently biocomposite-type implants. Polymer implants were first used in the early 1960s. Since then significant advances have been made in the composition of these implants, making them bioabsorbable while still providing adequate fixation. When compared with metal or nonabsorbable polymer implants, bioabsorbable materials have the advantage due to better postoperative imaging, excellent biocompatibility, decreased occurrence of osteolysis and tunnel widening, and lastly ease of revision.

First generation biodegradable interference screws were made of polyglycolic acid, which was absorbed too quickly and caused foreign-body reactions. Polyglycolic acid was replaced by poly- l -lactic acid (PLLA). However, PLLA required several years to fully degrade and was not replaced by bone. Additionally, PLLA degradation creates an acidic environment leading to local reaction and inflammation, subsequently inhibiting ossification.

Biocomposite implants consist of two different materials: a bioabsorbable polymer and a bioactive ceramic. The addition of bioactive ceramics such as hydroxyapatite, β-tricalcium phosphate (β-TCP), biphasic calcium phosphate, calcium carbonate, and calcium sulfate to bioabsorbable polymers increases osteoconductivity of the implant. Bioceramics release basic salts during degradation and likely buffer the acidic environment created by PLLA breakdown. Creation of a more basic environment decreases inflammatory response and promotes ossification. Even though addition of β-TCP to biocomposite implants is relatively new, β-TCP has been used for management of fractures and bone defect as void filler. β-TCP is a soluble phosphate ceramic, which with the help of osteoclast degrades into phosphate and calcium ions, creating a porous structure for bone ingrowth, promoting faster incorporation of screw into the adjacent bone.

Biocomposite interference screws need to provide adequate fixation during the immediate postoperative period and allow for healing of the graft as they become resorbed and replaced by bone. Several studies have evaluated degradation, resorption, and remodeling of PLLA/β-TCP screws, and all report superior results in comparison with other types of fixation. A study by Panagiotis et al. compared Ligafix 30 (30% β-TCP and 70% PLLA) and the Ligafix 60 (60% β-TCP and 40% PLLA) screws for resorption and remodeling in 25 anterior cruciate ligament (ACL) reconstructions. In this study, 37 months after implantation, a screw consisting of 30% β-TCP degraded more slowly than the 60% β-TCP screw, and while a 60TCP screw had a density reduction of 62%, 30TCP screw density was reduced by 52%. Additionally, 60TCP and 30TCP screws had similar density to surrounding bone and lower density than unimplanted screws. All patients had satisfactory clinical results comparable to other forms of fixation. The above-mentioned results indicate that higher β-TCP content accelerates absorption and promotes remodeling, while still offering adequate fixation strength.

A study by Barber et al. evaluated absorption, osteoconductivity, and clinical outcomes of a β-TCP-PLLA Bilok screw (75% PLLA and 25% β-TCP) in ACL reconstruction of 20 patients at a mean of 50 months after surgery. In all cases the Bilok screw was completely reabsorbed. Density of screw sites were close to that of bone and were significantly higher than densities of PLLA-only screw tracts reported in past studies. This finding demonstrated clear evidence of osteoconductivity and ossification of β-TCP-PLLA screws not seen in PLLA screws alone. In addition, no inflammatory reaction commonly seen with non-β-TCP-PLLA crews was detected. All patients had satisfactory clinical outcomes.

Recommendation for the use of β-TCP-PLLA-containing screws over other types of absorbable fixation is supported by clear advantage in terms of absorption, osteoconductivity, ossification, decreased inflammatory response, and adequate fixation strength.

Technique

ZimmerBiomet ComposiTCP screw is very simple and adaptable in terms of use. The screw can be used as interference fixation for multiple purposes around the knee. The set itself requires very few instruments. The authors generally recommend using a screw that is 1–1.5 mm greater than the tunnel which is drilled. Any bioceramic material has the potential to break or deform during insertion. The ComposiTCP screw is 60% tricalcium phosphate in sizes more than 8 mm in diameter. The screws in the 6-, 7-, and 8-mm diameters are 30% tricalcium phosphate. Along with this change of material, there is a change of associated tap and screwdriver.

The following set shows the instruments that are necessary for insertion of the ZimmerBiomet ComposiTCP screw ( Fig. 74.1 ).