This study was performed to investigate the long-term mechanical fixation stability of the angle-stable locking system using the new biodegradable sleeve in the treatment of distal tibia fractures fixed with IM nailing. Angle-stable locking results in a significant increase in torsional fixation stability including a significant reduction of the neutral zone, range of motion and increase of stiffness compared to conventional locking. Additionally the torsional stability showed no significant change over 12 weeks within both groups. For axial loading angle-stable locking had a less powerful impact on stability including stiffness and range of motion. Axial stiffness was significantly higher for the angle-stable group starting from week 6.
Intramedullary nailing of tibia fractures is an accepted and widely used treatment option. In the past few years this method has been used to address even more distal tibia fractures. This extension of the indications goes along with a growing number of reported complications such as delayed healing, nonunion, coronal plane and rotational malalignment/malunion [9, 16, 20–23].
Various modifications and new developments of implants have been introduced to reduce the complications and make the benefits of intramedullary nailing applicable even in these distal tibia fractures. First of all the number and the sites of the distal locking holes were adapted to the pattern of very distal tibia fractures. In biomechanical and clinical studies shortened tibia nails showed comparable biomechanical stability and good clinical results compared to standard nails [24, 25]. Modern intramedullary tibia nails allow distal locking using up to four screws.
One novel approach to the problems of distal tibia fracture management is the angle-stable locking of intramedullary nails with use of a preassembled 70:30 poly(L-lactide-co-D,L-lactide) sleeve on ASLS locking screws. A few previous studies already investigated this technique using a PEEK (polyetheretherketone) sleeve. In summary these studies showed a significantly higher axial stiffness and significantly less fracture gap movement  and a significant reduction of the neutral zone in mediolateral bending for the angular-stable locked intramedullary nails . A third study showed the potential of angle-stable locking to maintain fixation stability while reducing the number of locking screws in the treatment of unstable distal tibia fractures . All these studies used the PEEK sleeves. In a former study our group already investigated the primary biomechanical fixation stability of the new biodegradable sleeve. In this study we found the angle-stable locked constructs providing a significantly higher torsional and axial primary stability compared to conventional locking . In the present study we could affirm the advantage of the angle-stable locking especially for torsional fixation stability over a time of 12 weeks. In an in vivo study using tibia midshaft fractures in sheep Epari et al. could show the negative influence of torsional and sheer stresses on fracture healing . Additionally the group of Kaspar et al. found angle-stable locking of intramedullary nails in tibia fractures to result in less fracture gap movement and better radiologic, histomorphometric, biomechanical and clinical fracture healing in sheep . Therefore angle-stable locking of intramedullary nails seemed to be an option to reduce the risk of delayed union and, because of the increased stability, of secondary loss of reduction. Thus, this procedure potentially provides an option to use intramedullary nailing in even distal tibia fractures and osteoporotic fractures.
One concern about the use of biodegradable implants is an inflammatory reaction (seen after medial malleolus fixation). The system is available since 2009 and, to the knowledge of the authors until now no case of inflammation or problems in wound healing have been reported. The sleeve degrades to lactic acid; the degradation speed depends on the location of the sleeve and patient characteristics. The manufacturer (Synthes, Solothurn, Switzerland) guaranteed mechanical characteristics for four weeks. We have chosen the time-points for mechanical measurements from a clinical point of view, the healing process of a tibia fracture treated with an intramedullary nail can take up to 3 month (12 weeks). The huge differences between the sleeve and implants used for medial malleolus fixation are the small amount of material of the sleeve and the total intramedullary and nearly complete intra-nail location of the sleeve. This results in a very small interaction area between human body and biodegradable sleeve.
This study has limitations. First, the use of porcine bone, this material is widely used for biomechanical testing due to its availability , for example the knee and spine [32–35]. From these investigations we know that the bone mineral density of the porcine tibia is higher than human [36, 37]. For the interpretation of the results we have to take into account that we do not have an osteoporotic bone model. But the advantage of increased stability of the angle-stable locking is due to decreased screw-nail movement and will also be present in osteoporotic bone. Also the number of specimens was small, but it was high enough to show significant differences between the groups. Furthermore, the standard deviation in the AS group was very small. In the CL group it was clearly higher; this may be caused by the locking procedure: if the locking bolts are not placed ideal perpendicular and central in the locking hole they block the bone implant construct and a higher mechanical stability results. Such variables confound the results, but we tried to reduce them e.g. by standardized locking using a custom made drill and locking jig. Second, we used a “hydrolysis chamber” with standardized temperature and pH to simulate physiologic conditions. Although we know this is a model, it is a well-established way to simulate the in-vivo absorption (hydrolysis) of polylactid-polymers [38–42]. Furthermore, loading conditions have been chosen to allow subsequent torsional and axial loading over 12 weeks without causing plastic deformation due to testing. Thus biomechanical loading in this study does not represent physiologic conditions. Additional biomechanical studies investigating the biodegradable sleeve under physiologic loading have to be performed.
Clinical studies will be required to investigate the utility of the technique in the management of these difficult to treat distal tibia fractures and to show the benefit in patient care.