Future Directions - Articular Cartilage Tissue Engineering - page 87

Better clinical results have been seen for ACI than microfracture although both show satisfactory
outcomes at medium-term follow-ups [ 727 ].
Clinical studies involving joint repair have long established that immobilization inhibits long-
term healing in articular cartilage [ 728 - 731 ]. Motion is needed to induce movement of fluid and
nutrients throughout the joint spaces, as well as providing mechanical cues that can stimulate the
chondrocytic phenotype. However, the rigorous mechanical environment of the joint is too challeng-
ing for newly formed tissues, often resulting in rapid failure of the implanted constructs. Continuous
passive motion is commonly used for the first two weeks after surgery to facilitate the transport of
fluid, nutrients, and solutes within the joint, thereby stimulating chondrocyte metabolism [ 731 ].
Passive motion alone may be insufficient for cartilage healing, though, since it does not allow any
significant loading of the tissue [ 730 ]. Active motion, including incremental strength and weight
bearing exercises, may be necessary to stimulate repair processes during rehabilitation. As in many
in vitro experiments, chondrocytes respond best when suitable mechanical forces are present.
As discussed previously, cartilage resurfacing alone may be insufficient in completely address-
ing lesions as other problems may underlie the lesions' formation. The same can be said of therapies
using either mosaicplasty or ACI. As joint malalignment can result in lesions, the post-operative
joint alignment can affect ACI outcomes [ 732 ]. Other factors include concomitant treatments,
such as different rehabilitation regimens [ 733 , 734 ], and, of course, patient age [ 735 ] and surgical
history [ 736 ].
5.3.2.4 Osteotomy
The surgical procedures described thus far are oftentimes required due to lesions that result from
improper mechanics. In the cases of malalignment, contact pressures of the defect area may be reduced
to physiological levels by osteotomy. In this case, methods include anteromedial transfer of the tibial
tubercle to decrease the contact forces on the lateral facet of the patella [ 737 ], as anteromedialization
of the tibial tubercle has been shown to lower contact pressures in the lateral trochlea in a cadaver
model [ 738 ]. This, in combination with other surgical techniques such as ACI, can be beneficial
to patients with multiple knee disorders [ 739 ]. Currently, the combined consideration of alignment
with autologous osteochondral grafting are associated with traumatic reconstruction case studies
and are thus difficult to compare [ 740 ]. Nonetheless, in order to prevent relapse, future cartilage
resurfacing methods may consider the accompaniment of additional orthopaedic adjustments.
5.3.2.5 Other Treatments and EmergingTechniques
An alternative to in vitro tissue engineering is to implant a scaffold in vivo , with or without cells, and
allow regeneration to occur with minimal additional manipulation. Regeneration of hyaline cartilage
within the body is complicated by the rigorous mechanical environment present in active joints.
However, researchers are currently investigating means to accomplish this goal since overall healing
time is anticipated to be less for in vivo versus in vitro tissue engineering approaches. Furthermore,
the complex mixture of biochemical and biomechanical cues present in the body can accelerate tissue
growth that is difficult to produce otherwise. Most of the factors previously discussed for in vitro
tissue engineering are applicable to in vivo tissue engineering, except for some types of mechanical