Implants in Sports Medicine



Implants in Sports Medicine


F. Alan Barber





DEFINITION



  • Arthroscopic surgical techniques in sports medicine have significantly advanced as a result of the development of various implants to repair ligaments, tendons, and meniscus cartilage. These anchoring devices are currently combined with ultra-high-molecular-weight polyethylene (UHMWPE)-containing suture.


  • These anchors and their sutures are used in the glenohumeral joint for labral and ligamentous attachment to the dense glenoid bone, in the bursa for rotator cuff and biceps tendon attachment to the greater or lesser tuberosity, and in the knee to repair tears in torn menisci.


  • Anchor designs are procedure specific because of the different repair requirements and techniques. Some designs contain multiple sutures and are most suitable for osteoporotic bone (rotator cuff repairs). Others contain fewer sutures and are better in denser cortical bone (glenoid repairs). Meniscal repair devices have smaller sutures, and implants are designed to approximate and hold torn meniscal tissue.


ANATOMY



  • The principal application of all repair implants is to secure the tissue (tendon, ligament, or meniscus) to the appropriate site without excessive tension, in a way that resists loosening, and which allows for physiologic healing.


  • Rotator cuff



    • Repair concepts for the rotator cuff that have facilitated the goal of an effective repair include an appreciation of margin convergence,19,31 the deadman angle for anchor insertion,18 orthogonial repair orientation,34 and understanding appropriate anchor location. Controversy exists about the clinical effectiveness of complete footprint coverage and the posterior interval slide.28,36


    • Several options exist for placing anchors regarding the footprint attachment. These include locating the anchors at the edge of the humeral articular cartilage to reduce the tension on the tendon, locating the anchors more lateral to the articular cartilage but still on top of the tuberosity, locating the anchors on the lateral side of the humerus in the cortical bone shaft (referred to as orthogonal)24,25,32,33 and using bridging sutures to compress the cuff tendon to the prepared tuberosity,20,21 and using a double row of anchors to secure the tendon adjacent to the articular cartilage and at the peripheral greater tuberosity.


    • The direction or angle of anchor insertion is especially important for rotator cuff repairs. The commonly recommended angle for anchor insertion is the deadman angle (45 degrees).18 However, the 45-degree angle is actually the maximum acceptable angle for insertion rather than the ideal. More acute insertion angles are often better. The humerus adjacent to a chronic cuff tear often has an osteoporotic (“hollow”) humeral head with fewer trabeculae than normal. Directing the anchor in a more tangential, superior angle places the anchor into the denser subchondral bone.


    • Anchor depth is important. Anchors inserted too deep fail by the suture cutting through the bone or by anchor displacement by rotating and translating toward the cortical surface. An anchor that stands “proud” is more likely to fail at the eyelet.


  • Glenoid



    • Glenoid bone is denser than that in the greater tuberosity. Also, working in the restricted space of the glenohumeral joint places limits on the size of implants. As with rotator cuff repair, various portals are used to reach different anchor insertion locations, and careful consideration of proper portal positioning is important. For insertion angles, one should keep in mind the anatomy of the glenoid to avoid breaking through inferiorly, avoiding injury to the axillary nerve about 1 cm from the 6 o’clock position and avoiding placing the anchors too medially on the anterior face of the glenoid. Anchor placement should be on the articular cartilage near the edge and not down on the neck.


  • Meniscus



    • The anatomic features related to meniscus repair deal with both the blood supply and the tear type.


    • The meniscal blood supply comes from the periphery, which is more vascular than the more central areas.2 A good vascular supply is essential for healing; therefore, selection of a meniscus tear for repair must realize that repairs in the more central area are unlikely to heal. The meniscus has been divided into three zones based on vascularity.3 The peripheral third (“red/red zone”) has the most vascularity. The less vascular (“red/white zone”) middle third is an area where repairs may be considered if no degenerative changes are observed in the meniscus. The avascular inner third of the meniscus is called the white/white zone and will not heal.


    • Tear type also plays a role. Degenerative tears will not heal. Indications of degeneration include fraying of the meniscus, multiple tear planes, rolling of the inner fragment when probed, and a chronically locked displaced bucket handle.


    • The most common location of repairable meniscus tears is the posterior third. Reaching this area in a tight medial compartment can be a challenge and lead to chondral excoriation with repeated passes of the device.


    • As the popliteal tendon passes behind the posterior lateral meniscus, a hiatus is created with no attachments to the
      lateral meniscus. This creates a challenge for repair should the tear extend into and through this region. Placing sutures through the meniscus and into the popliteal tendon is not advised.


    • Risks associated with meniscal repair include vascular damage to the popliteal artery, chondral injury, and soft tissue entrapment.22,23


MATERIAL PROPERTIES



  • Suture anchors



    • Historically, anchors were made from various metals, including stainless steel and titanium. More recently, nonmetallic anchor have been introduced made from plastic (PEEK), biodegradable (PLLA, PDLLA, or PLA-PGA), and biocomposite (containing beta-tricalcium phosphate or hydroxyapatite) material.6,10,12 Biodegradable anchors demonstrate comparable pullout strength, degrade completely over time, and avoid revision surgery or postoperative imaging problems. Recently introduced biocomposite materials offer osteoconductive behavior, leading to anchor replacement by bone at the end of the degradation process.10


    • Biodegradable and biocomposite suture anchors are just as effective as nonabsorbable anchors. Degradable polymers commonly used in suture anchors include polyglycolic acid (PGA), poly-L-lactic acid (PLLA), stereoisomers of lactide such as poly-D-L-lactic acid (PDLLA), and combinations (copolymers) of lactide and glycolide (PLA-PGA). The slowest degrading polymer used in implants is PLLA, which takes years to reabsorb.9 In an effort to reduce the time needed for an implant to degrade, various stereoisomer combinations of PLLA (PD[96%]L[4%]LA or PD[70%]L[30%]LA) have been introduced as well as copolymers (PLLA co-PGA), which as yet have not been associated with a lytic response.


    • Polyetheretherketone (PEEK) is a nonabsorbable, biologically inert polymer. PEEK is a chemically resistant organic crystalline thermoplastic polymer adaptable to a wide pH range from 60% sulfuric acid to 40% sodium hydroxide which can resist deformation even at high temperatures. It can be combined with carbon fiber for reinforcement and has many current applications in orthopaedic surgery beyond suture anchors.


    • Although a PEEK anchor can be drilled through during a revision procedure, this process would create many small plastic shavings, which could be thrown into the joint and become difficult to remove. Such PEEK shavings will never degrade and create an abrasive, which could injure the articular cartilage.


    • Patient age is a factor. Older patients may be better choices for nonabsorbable devices, but the patient typically undergoing shoulder instability surgery is young. Using a degradable anchor is attractive because of the patient’s anticipated longevity.


    • Biocomposite anchors reflect a significant advance in material technology. Biocomposite materials are combinations of a degradable polymer with a bioceramic. Combining biodegradable polymers and beta-tricalcium phosphate (β-TCP) creates a blend, possessing attractive properties of both materials. For instance, the compressive strength and stiffness of β-TCP is very high and, when blended, imparts these characteristics to the biocomposite. The resulting material degrades over time and stimulates osteoconductive ingrowth of bone into the space previously occupied by the anchor.8,10


    • These nonmetallic materials are radiolucent and can be drilled through during a revision procedure. Those which do not absorb over time present the same concerns as any metal anchor.


    • Recently, anchors composed entirely of suture have been introduced.12 One or more strands of braided UHMWPE suture is joined with a short sleeve of braided polyester or UHMWPE material, which creates an anchor after being inserted into the bone. Traction on the suture bunches the sleeve, creating a ball of material that forms the anchor within the bone.


  • Meniscal repair devices



    • Common characteristics of the latest generation of all-inside devices include the use of UHMWPE suture material and a self-adjusting, locking knot, which does not require the addition of half hitches for knot security. Although some previous versions of these devices used small anchors made from biodegradable polymer, most of all the current versions use anchors made from PEEK. The exception is a completely suture based repair device which inserts two parallel tubular needles into the torn meniscus after which a small shuttling needle transfers the repair suture between these two parallel needles to create a horizontal repair stitch.


  • Sutures



    • Sutures can be physically described as monofilament, braided, or blended and absorbable, nonabsorbable, or partially absorbable. The most common completely biodegradable monofilament suture used is polydioxanone (PDS). It is frequently used in glenohumeral instability surgery. Although any type of suture can also be used to shuttle braided sutures through tissue, the monofilament characteristic of PDS allows it to work with suture hook devices in a way that braided sutures cannot. It also can be used as a marker stitch to facilitate the identification of a rotator cuff tear in the bursa. PDS suture degrades quickly; it retains only 60% of its original strength 2 weeks after implantation, 40% by 6 weeks, and is almost completely reabsorbed by 9 weeks.


    • Nonabsorbable braided polyester sutures, such as Ethibond, were, until the development of UHMWPE sutures, the suture of choice for most soft tissue repairs and in suture anchors. However, braided polyester has been replaced in most arthroscopic applications and in all current suture anchors by high-strength UHMWPE-containing suture. FiberWire (Arthrex, Inc., Naples, FL) was the first high-strength suture and consists of a braided polyester coat surrounding a central core of multiple strands of UHMWPE.


    • The commercial availability of FiberWire redefined expected suture strength. This lead to the subsequent introduction by competitors of sutures made from pure braided UHMWPE. A single manufacturer provides braided UHMWPE suture for other companies, and it is currently marketed under several different brand names. The pure braided UHMWPE suture has almost twice the ultimate strength of FiberWire (which is partially braided polyester) and a 500-fold increase in resistance to fraying compared to pure braided polyester suture.15



    • OrthoCord (DePuy Mitek, Raynham, MA) is the most recently introduced high-strength suture and is used in DePuy Mitek suture anchors. OrthoCord combines both UHMWPE suture with degradable PDS suture. No. 2 OrthoCord consists of 32% UHMWPE and 68% PDS and is coated with polyglactin 910. OrthoCord has a PDS core with a UHMWPE sleeve and leaves a lower profile after the PDS reabsorbs while retaining the outer sleeve strength.


Suture Anchors



  • Many ways exist to classify shoulder suture anchors. For this chapter, they will be classified as medial row anchors, lateral row anchors, and glenoid instability anchors.


  • Medial row anchors



    • Medial row anchors tend to be more robust and have higher load to failure strengths than the other two types. These anchors are often also used for biceps tenodesis. These anchors withstand the higher biomechanical stresses found at the medial side of the rotator cuff footprint, which can measure over 900 N in some instances in the infraspinatus. Medial row anchors are usually screwin anchors and require knot tying.


  • Lateral row anchors



    • In contrast, lateral row anchors are usually knotless designs.6,29 The knotless designs have been shown to be clinically effective by themselves. Recently, their principal application relates to their use for a suture bridge technique in which sutures from medial row anchors, after being passed through the tendon in a mattress stitch fashion, are tied and then fixed laterally in the knotless anchor to compress the remaining tendon to the greater tuberosity, creating a more expansive footprint of attachment. This configuration applies pressure on the rotator cuff tendon and compresses the tendon against the greater tuberosity bone bed during healing. Some advocate placing the lateral row anchors “over the top” on the lateral side of the greater tuberosity parallel to the cuff tendon. This “orthogonal” or “anatomic” anchor position is felt by its proponents to be superior to placing the anchor in a “deadman angle” at the edge of the greater tuberosity.18,34


  • Glenoid instability anchors



    • Glenoid instability anchors are principally designed for shoulder stabilization procedures in younger patients with better bone quality than those undergoing a rotator cuff repair.


    • Instability rehabilitation programs generally call for a period of immobilization, which allow the capsule and glenohumeral ligaments to be well on the way to healing before the stresses of rehabilitation are applied. The capsulolabral tissue and bone at a shoulder instability repair are younger and healthier than that encountered in rotator cuff tendon repairs. Consequently, the biomechanical properties and design features of an acceptable glenoid anchor will be different from one used in the humeral tuberosity.


    • Glenoid anchors are smaller, have lower profiles, and are designed to be inserted into cortical bone. Glenoid anchors range in size from under 2 mm in diameter up to 3.5 mm. This smaller size meets the requirements of the confined space and dense glenoid rim. Toggle anchor designs generally ineffective for a decorticated osteoporotic greater tuberosity are applicable in the glenoid. Smaller and shorter anchors can be accommodated in the glenohumeral joint’s smaller space. Shorter anchors avoid overpenetration at the inferior glenoid, which could lead to breaking out of the bone into the axillary space and potentially injuring the axillary nerve.


    • However, these smaller anchors have lower failure loads than the larger rotator cuff tendon anchors. Smaller glenoid anchors cannot accommodate as many sutures as the larger cuff anchors, and this must be considered in selecting the appropriate anchor for glenoid capsule-ligamentous repair. Both knotless and knot-tying glenoid anchor designs are available.


Meniscal Repair Devices



  • The latest generation of meniscal repair devices allow for an all-inside technique. The initial generation of all-inside devices provided rigid fixation (ie, tacks, staples, and screws) and lower load to failure strength and carried risks of chondral abrasion because a portion of these devices remained exposed on the meniscal surface.


  • The development of “self-adjusting” all-inside meniscal repair devices offered greater strength and safety, but using braided polyester suture material were still subject to breaking.


  • The current generation suture-based, all-inside, self-adjusting meniscal repair devices containing UHMWPE suture provide a stronger all-arthroscopic technique, which is less likely to break, avoids the need for additional incisions about the knee, and decreases the potential for injury to the neurovascular structures about the knee.16


  • Common characteristics of all these devices include the use of UHMWPE suture material and a self-adjusting, locking knot, which does not require the addition of half hitches for knot security. All of these devices are inserted into the meniscus using a needle, and then once at least two passes of the suture are in position, the knot is tensioned and the repair secured.


  • Differences exist in the instrumentation, design, suture size, implant size, and type of knot deployed for the different devices. Comparisons of these devices often evaluate load to failure strength, mode of failure, and other mechanical properties.


Sutures and Knots



  • Arthroscopic sutures should possess good handling characteristics, good strength, good loop and knot security, and, when appropriate, be biodegradable. If degradation should occur, the suture should not create a significant inflammatory response. Furthermore, a superior arthroscopic suture offers greater strength for its size while maintaining a low-friction surface conducive to tying in the wet, arthroscopic environment.


  • Concerns exist about knots tied with these UHMWPE sutures. Reports exist that such knots are susceptible to slipping before breaking at loads below expected failure loads.1,13 This is due to the physical properties of UHMWPE suture and the type of knot being tied.


  • Some knots are more susceptible to slipping when tied using UHMWPE suture than others.1,13 The Duncan knot and Weston knot were reported to slip at submaximal loads in 97% and 86% of the time, respectively. In contrast, the SMC knot and the Revo knot slipped only 1% and 3%, respectively, at a submaximal load. The San Diego knot and the Tennessee slider knot were reported to slip less than 10% of the time in that same study.13 Therefore, using UHMWPE
    suture may provide a higher strength suture but not necessarily a higher strength knot. This greater risk of knot slippage can be mitigated by choosing the right knot.


  • Knot types and uses



    • There are two types of arthroscopic knots: sliding knots and nonsliding knots. As surgeons, we should be familiar with knot security and loop security.


    • Knot security is the ability of the knot to resist slipping when a load is applied. Three factors can affect this: friction, internal interference, and slack between throws.


    • Loop security is the ability of maintaining the size and tension on the loop during knot tying.


    • It is possible to have a loose knot on a secure loop (poor knot security) and it is possible to have a secure knot on a loose loop (poor loop security). Either construct will be ineffective in tissue repair.


    • Sliding and nonsliding knots



      • All arthroscopic knots (both sliding and nonsliding) start with a foundational knot that removes any slack at the tissue interface. This is then secured by several additional half hitches. Sliding knots start with a specific locking hitch (created outside the joint), whereas nonsliding knots create that locking hitch with a series of half hitches (created inside the joint).


      • To counter the problem that arthroscopically only asymmetric tension can be applied to the two suture strands creating a less secure knot (not square throws), complex sliding locking knots have been developed. These knots develop internal resistance and then lock, resulting in better knot and loop security.


    • Locking and nonlocking knots



      • Nonlocking knots (Duncan loop) are held in place by the friction of the suture as the knot is tightened. The UHMWPE suture has less friction, and this holding requirement is not consistently met.


      • Locking knots (such as the SMC, Tennessee slider, San Diego, and Weston) have an internal locking mechanism such that when the non-post limb of suture is tensioned, the knot changes its configuration and locks in place. The surgeon will feel the knot locking by a snapping or clunking sensation in the sutures. Once locked, the knot cannot be moved. It is important to make certain that the knot is correctly positioned before locking it.


FAILURE MECHANISMS

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Jul 24, 2016 | Posted by in ORTHOPEDIC | Comments Off on Implants in Sports Medicine

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