Mylène Jansen

198 Chapter 10 reasoning behind the role of unloading in the clinical and structural benefit of distraction (and other partial unloading techniques). The actual strain magnitudes on cartilage are important for the fate of cartilage and the chondrocyte response. During normal activities, diurnal strains range from 0–10% 47,48 , post activity strains range from 5–15% 49–52 , and dynamic strains during activity range from 15–35%. 53,54 At higher nominal strain magnitudes (50–70%), mechanical compression can cause injury 55–57 , eventually inducing cell death via necrosis and apoptosis at strains of the highest levels (70–90%). 58,59 Effects of different loading conditions influence the chondrocyte function. Static loading decreases cartilage metabolic activity 60 , physiologic levels of dynamic loading can be anabolic or anti-inflammatory 61–64 , while hyper physiologic levels of dynamic loading and injurious-loading can induce catabolic or pro-inflammatory responses. 62,65,66 Moreover, controlled impact experiments on cartilage tissue indicated that shear stress, rather than impact force, is the strongest predictor for the occurrence of cartilage damage (fissures). 67 The damaged cartilage in OA might result in a disturbed perception of normal weight-bearing and thereby direct chondrocytes to further dedifferentiation resulting in further degeneration. This all suggests that treatments where the strain magnitude and shear stress shift from severe and are (temporarily) reduced, result in a favorable mechanical environment to allow repair. With joint distraction, loading of cartilage and bone is reduced. The most recent insight made clear that the intended mechanical unloading is only partial. During treatment patients are encouraged to walk and load their distracted joint. In most instances there will be most likely contact between the articular surfaces during loading of the joint due to resilience in the distraction frames (personal observations from mechanical bench testing of different devices; manuscript in preparation). This is supported by a previous study demonstrating that the joint surfaces of cadaveric ankles undergoing distraction contact in case of 5 mm distraction under 70 kg loading. 68 Irrespective, a condition is created where mechanical overload (strain and shear), an important driving force of joint degeneration, is temporally neutralized. Moreover, in case of non-articulating devices, shear stresses will be fully absent. This absence of mechanical wear and tear on the cartilage is considered to be of importance. Chondrocytes are sensitive to mechanical stimuli to maintain cartilage integrity. 69 This means that complete immobilization and absence of any chondrocyte mechanical stimulation may be ineffective. At present it is unknown to which degree (force and frequency) mechanical contact of both cartilage surfaces during loading of the distracted joint is a prerequisite or parameter to facilitate cartilage tissue repair. More or less deviated from an optimal condition may be related to more or less or less or more repair. The exact force and frequency will be highly variable between patients in daily practice and may be an explanation for the variable joint tissue repair activity seen in the clinical studies. Future studies, taking this parameter into account by recording and controlling resilience in the distraction device, weight of the patients, and actual loading of the joint during distraction are warranted to get a grip on this parameter and potentially improving repair activity in case an optimum can be reached for each patient.