Introduction

The initial interference fixation screws were made from metal and provided screw fixation for anterior cruciate ligament (ACL) reconstructions. Biodegradable interference fixation screws subsequently gained wide acceptance after their introduction in the early 1990s.^1–5 The benefits of these biodegradable interference screws include reduction in the concerns previously associated with metal implants, including difficulties in postoperative imaging, reduced graft laceration during insertion, less chance of screw divergence during insertion, easier revision surgery (Fig. 51-1), and fewer problems with secondary arthritic procedures that might require the complete removal of a metal screw. In addition, the load to failure (LTF) strength of these screws is sufficient to allow for an aggressive rehabilitation program. Potential complications associated with biodegradable interference screws include the risk of screw breakage during insertion, decreased holding strength when compared with a metal alternative, and inflammatory reactions that could lead to lytic changes and cyst formation.

Fig. 51-1 Biodegradable interference screws avoid some of the problems during revision surgery that are associated with the prior placement of metal screws.

Poly-L-lactic acid (PLLA) is the most common material used in biodegradable interference screws. These screws offer effective graft fixation with little evidence of adverse inflammatory reactions, and many years pass before any material degradation occurs.^1–8 Interference screws made of lactic acid copolymers containing dextro and levo stereoisomers subsequently became available, as did copolymers of polylactic acid and polyglycolide. The polymer degradation is more rapid with poly D, L-lactide (PDLLA) implants than pure PLLA in animal studies.^9–11

The Milagro screw (DePuy Mitek, Norwood, MA) is composed of a composite of 30% osteoconductive beta tricalcium phosphate and 70% polylactic glycolic acid by weight and represents a new material for interference fixation screws. This review presents biomechanical data, an explanation of the material properties of the screw composition, and a discussion of the clinical technique.

Biomechanical and Biochemical Data

Implant degradation proceeds through five stages: hydration, depolymerization, loss of mass integrity, absorption, and elimination. How rapidly an implant is degraded is influenced by the polymer of which the implant is made, the degree of crystallization of that polymer, the initial mass of the polymer present (implant size), surface coverings, whether the polymer is self-reinforced, the processing technique used (machining or injection molding and sterilization technique), and the environment in which the implant is situated.¹² In addition, the degradation mechanics of different polymers may differ considerably based on the hydrophilic or hydrophobic nature of the different polymers.

Degradation starts at the amorphous phase of the implant and leads to fragmentation of the material to smaller parts, which are then phagocytosed primarily by macrophages and polymorphonuclear leukocytes.^13,14 The lactic acid component is broken down by hydrolysis. The resultant monomers enter the Krebs cycle and are further dissimilated into carbon dioxide and water.¹⁵ In addition to hydrolytic chain scission, glycolic acid monomers are degraded by the enzymatic activity of esterases and carboxypeptidases.¹⁶

All biodegradable materials cause some inflammatory response. The longer the degradation course, the less visible the response will be. Usually there is a mild, nonspecific tissue response with fibroblast activation and the invasion of macrophages, multinucleated foreign body giant cells, and polymorphonuclear leukocytes during the final stages of degradation. Because of the more rapid degradation associated with polyglycolic acid (PGA), there have been some foreign body reactions with varying degrees of severity ranging from mild osteolytic changes to intense granulomatous inflammatory soft tissue lesions that necessitate surgical intervention.^17,18

Concerns about implants composed of pure PGA have led to the development of PGA copolymers that still have a more rapid rate of absorption compared with PLLA implants, but the literature supports their use with excellent clinical results. Lajtai et al^19,20 reported good results with a lactide/glycolide copolymer screw (85/15, D, L lactide/glycolide). Using magnetic resonance imaging (MRI scans), the screw was shown to remain intact for 4 months and then disappear by 6 months. Five years after implantation, the screw was completely reabsorbed and evidently replaced with new bone. Morgan et al²¹ evaluated a PLLA interference screw removed en bloc from a patient 2.5 years after insertion. The histological examination and molecular weight measurements showed a 75% decrease in the molecular weight of the screw with implant fragmentation and new bone formation adjacent to the screw. This dramatically contrasted with the MRI evaluation of the patient, which showed the presence of a clear screw outline. MRI evaluations of PLLA screw show no evidence of any progressive absorption 4 years after implantation.²² However, a recent computed tomography (CT) evaluation of patients who underwent patellar tendon autograft ACL reconstruction using PLLA screws at least 7 years earlier demonstrated complete removal of the PLLA screws without any significant bone ingrowth into the screw site.²³

The goal of using biodegradable polymers is to have an implant mechanically strong enough to perform its task and then degrade in a manner that is clinically insignificant. An additional advantage would be that once the degradation is complete, there would be no evidence of the implant ever having been in place. At this point, pure PLLA and copolymers of PLLA and PGA have demonstrated adequate strength as interference fixation screws to function effectively for ACL reconstructions. These PLLA and PLLA/PGA copolymers have also demonstrated that they will eventually degrade and disappear, even though it may require many years for this to occur (longer for PLLA than for PLLA/PGA copolymer). The next step is to develop an implant that will result in bone filling the vacated screw site.

Basic Science of Beta-Tricalcium Phosphate copolymers

Bone replacement technology has been in development for many years. Calcium phosphate ceramic materials like beta-tricalcium phosphate (ß-TCP) have been studied as potential bone replacement materials for decades. The calcium phosphates are used as bone void fillers, autograft extenders, and coatings for various implants including joint replacements. They are also used in products in which reabsorption of the device and replacement with native bone are desired, including different orthopaedic and maxillofacial applications.

Bone, as with other calcified tissue, is an intimate composite of organic (collagen and noncollagenous proteins) and inorganic or mineral phases. Bone has several important properties including osteoconductivity (the ability to serve as an interactive template or scaffold for forming new bone) and osteoinductivity (the ability to create new bone or osteogenesis). None of the current manufactured materials has the ability to form bone (osteoinductive), but the benefit of a material being osteoconductive and being able to act as a template into which the adjacent bone may migrate is clear. A biodegradable interference fixation screw with osteoconductive properties would enhance bone ingrowth into its location as it biodegrades.

Composites are a blend or intimate mixture of two different materials. This blending usually imparts different properties to the composite than those that were possessed by either of the two separate materials individually. The compressive strength and stiffness of ß-TCP are very high and, when blended with PLLA, the resultant composite includes these properties as well. How well dispersed the two materials of a composite are with one another is another important property. Once blended, the more homogenous the composite, the better. Biocryl is a composite of ß-TCP and PLLA. Biocryl is a very homogenous composite with a high degree of dispersion of both materials in the blend. This dispersion of the materials is achieved throughout the entire implant by a proprietary manufacturing process known as micro particle dispersion (MPD). The addition of polyglycolide to polylactide creates a copolymer that biodegrades much more rapidly than even a very amorphous form of pure PLLA. The Milagro screw is made of a material that combines the ß-TCP–PLLA (Biocryl) composite with PGA. This resultant compound polymer composite consists of 30% osteoconductive ß-TCP and 70% polylactide co-glycolide (PLGA). The presence of the ß-TCP encourages bone to fill in once the PLGA has reabsorbed.

Clinical Information

The Milagro screw can be used for femoral or tibial fixation for soft tissue or bone–tendon–bone (BTB) autografts or allografts (Fig. 51-2). It is available in various diameters from 7 to 12 mm and in 23-, 30-, and 35-mm lengths. The Milagro screw is made from a polymer composite, Biocryl Rapide. As previously mentioned, this material consists of 30% osteoconductive ß-TCP and 70% PLGA. The poly (lactide-co-glycolide) copolymer is composed of 15% PGA and 85% PLLA. This ratio of PGA to PLLA was chosen following animal studies to allow for a faster yet controlled absorption. This copolymer does not contain any of the D-isomer of lactic acid.

Fig. 51-2 The Milagro screw can be used for femoral or tibial fixation for soft tissue or bone–tendon–bone autografts or allografts.

This material was recently evaluated in the lateral femoral cortex of mature beagle dogs.²⁴ Rods of either Biocryl Rapide or PLLA measuring 3 × 10 mm were inserted into defects in the cortex and evaluated at intervals up to 24 months. Histological evaluation at intervals looked for reabsorption of the material and cracks, cell infiltrations, erosions, and fragmentation of the implants. At 24 months postimplantation, clear differences existed between the PLLA rods and the Biocryl Rapide rods (Fig. 51-3). The reabsorption of the Biocryl Rapide was nearly complete at 24 months, and radiographic bridging was observed at the Biocryl Rapide sites but not at the PLLA sites. No evidence of inflammatory reaction or cellular necrosis was observed. By 24 months, the entire cross-section of the Biocryl Rapide test rods was absorbed and replaced by normal bone or bone plus fibrous tissue or adipose tissue. The circular orientation of the new bone was seen under polarized light.

Fig. 51-3 At 24 months postimplantation, the reabsorption of Biocryl Rapide rods was nearly complete and radiographic bridging was observed.

Clinically, the Milagro screw has been available since its introduction in October 2004. The Milagro screw can be used for both BTB autografts and allografts. In these cases the technique for insertion is essentially the same as for any other biodegradable BTB interference screw. The preferred length for BTB fixation is 23 mm, with the diameter of the screw reflecting the size of the tunnel drilled and the size of the bone plug. I most frequently use the 8-mm-diameter screw for both the femoral and tibial sides; however, on occasion a 9-mm screw will be required for the tibial fixation.

Soft tissue grafts (hamstring allografts and autografts and the tendon side of a quadriceps tendon autograft or Achilles tendon allografts) require a longer interference interface between the soft tissue and the tunnel, especially in the tibial tunnel. For the hamstring or quadriceps soft tissue grafts, the 35-mm screw length is preferred.

Once the tunnels are drilled and the graft is prepared, a groove is made in the superior area of the bone tunnel with a notcher for the subsequent placement of a guidewire. The graft is then pulled into the tunnels. Once the graft is in place, a guidewire is placed in this notched groove, which is now adjacent to the bone plug in the femoral tunnel. Using the guidewire, a tap is inserted and threads cut to the correct depth. If the bone is softer, it is only necessary to cut a few threads, which allows the Milagro screw to engage the bone and then cut its own way into the interference position. For denser bone, full tapping for the entire screw length is needed.

The tap for the Milagro screw has distal threads that are blunt, whereas the more proximal threads are sharp. Care should be taken to avoid cutting the control sutures in the bone plug with these proximal tap threads during this tapping procedure. It is fairly easy to avoid cutting the sutures in the femoral plugs, but a greater awareness of the tibial plug orientation, its sutures, and where the tap is being inserted relative to both of these is required on the tibial side to avoid cutting the control sutures.

Once tapping is complete, the Milagro interference screw is advanced over the guidewire to the appropriate depth. Resistance during insertion is expected and is felt to increase as the screw advances. An increasingly loud squeaking should be heard as the screw nears the fully seated position. Our experience is with the 8- and 9-mm-diameter sizes. We have not had the Milagro screw break when the tapping was successfully done. One case of screw breakage occurred when the tapping step was skipped. The thread depth as measured between the minor (or core) diameter and the outer diameter is better than other biodegradable screws in our experience, and the thread pitch (number of threads per length) provides sufficient spacing between the threads to grip and compress the adjacent cancellous bone.

Postoperatively an aggressive rehabilitation program is followed. The speed and details of this program are dictated by the nature of the graft material selected and not by the presence of a biocomposite screw. The patellar tendon autograft reconstructions begin with maintaining full extension by prone hangs and a full-extension night brace. The knee flexion is encouraged by a constant passive motion machine for 6 to 8 hours during the day for as long as 2 weeks or until comfortable flexion to 90 degrees is attained. Straight-ahead jogging without cutting is started at 6 weeks postsurgery and pivoting at 12 weeks postsurgery. Full contact is begun with a derotational knee brace between 12 and 16 weeks after surgery. Allograft and soft tissue graft reconstructions are returned to activity less aggressively.

Because of the biocomposite in the Milagro screw, its position can be evaluated on postoperative radiographs. Postoperative radiographs are obtained on the first postoperative visit and serve as a baseline for subsequent evaluations. The ability to visualize the screw helps with assessing plug location.

Pearls

The tap associated with the Milagro screw has two types of threads. The distal threads are blunt, whereas the proximal threads are sharp. If the entire course of the screw is to be tapped, care should be taken that these cutting threads do not damage either the graft or the sutures attached to a bone plug. To reduce the chance of cutting the sutures, make certain that the guidewire is placed on the side of the plug between the two sets of sutures controlling the graft, and not through the sutures.

Once the tibial screw is in place, tug on the graft sutures to demonstrate that there is good tension on the graft and no movement of the graft in the joint. If graft movement is observed, the graft should be retensioned and a second screw stacked beside the first to achieve secure fixation (Fig. 51-4).

Fig. 51-4 If secure fixation of the tibial graft is not achieved with a single screw, a second screw may be stacked beside the first to achieve secure fixation.