208 Surgical Treatment
ligament replacement. A report describing
human semitendinosus grafts assessed a single
acute 80 N pull, a static 88 N pull for 20 min-
utes, or 20 cyclic pulls, each of 80 N. Cadav-
eric testing showed that either the extended
load or cyclic load was superior for prevent-
ing initial elongation (Piliaet al. 2015). In con-
trast, a study with a similar protocol found no
benefit when the secured grafts were tested in
using cyclic loading (Boguszewskiet al. 2015).
No studies have been performed in dogs for
CrCL graft placement, although it has been sug-
gested that preconditioning a CrCL allograft
would be important to eliminate initial elon-
gation (Yahia & Drouin 1990). Stifle angle at
the time of fixation could also affect tension. A
study in dogs showed that the fixation angle
had a greater effect on graft tension than tunnel
location (Shelleyet al. 1996). Although the ideal
angle was unknown, the study suggested that
stifles should not be flexed to 90◦at the time of
fixation.
Finally, graft fixation methods are varied, and
the optimal method continues to be debated.
In humans, multiple techniques for securing
bone–ligament and soft tissue grafts have been
described, including interference screws, pin-
ning, spiked washers, and suture (Hillet al.
2005). Interference screws are commonly used
in human surgery, and when used in a repair
technique they reportedly have an in vitro
strength of between 463 and 1358 N (Kousa
et al. 1995; Ruppet al. 1997). Possible benefits of
interference screws include ease of use, and that
they can be sized to accommodate grafts with
different cross-sectional areas, which improves
the contact between the bone tunnel and graft.
When using grafts that are folded to double
the graft, such as the hamstring tendon or
deep digital flexor tendon (DDFT), a cross-pin
technique is an alternative fixation technique
(Harilainen & Sandelin 2009). In cross-pin trans-
fixation techniques, a pin is passed through the
loop created at the fold in the graft. The remain-
ing graft ends can be secured with one of the
above-mentioned techniques.
A unique challenge in the dog is that the
initial fixation strength has to overcome daily
forces encountered by the CrCL, as a grad-
ual increase in weight-bearing is not possible
as in humans. Inex vivocanine stifles, it was
shown that an entire patella tendon secured
with interference screws or with bone anchors
was likely not strong enough to withstand daily
physiologic loads (Biskupet al. 2015). However,
when DDFT was secured with a cross-pin and
spikes washers, the mechanical strength was
similar that of the native CrCL (Figure 26.3).
Similarly, a novel graft fixation device has been
shown to have equal strength to spiked wash-
ers (Lopezet al. 2007). In veterinary medicine,
previous reports of commonly performed intra-
articular repairs showed grafts to be secured
most frequently only with suture (Baconet al.
1984; Coetzee & Lubbe 1995). Suture stabiliza-
tion of a graft undoubtedly provides insufficient
strength for patient use.
Fixation methods continue to progress in
human and veterinary medicine. The unique
challenges with management of canine patients
immediately postoperatively must be consid-
ered when choosing a method. Further studies
are needed to examine the effect of tunnel loca-
tion and graft tension on healing and clinical
outcome.Biology
Although it is believed that CrCL graft and fix-
ation mechanics must be sound, intra-articular
graft placement also creates biological chal-
lenges, including the obstacle of intra-articular
healing, promotion of vascular ingrowth
and remodeling, and avoidance of immune-
mediated rejection.
It is important to understand that intra-
articular graft healing differs from the heal-
ing of other extra-articular ligaments. Experi-
ments have evaluated the ability of the ACL in
humans to heal compared to the healing poten-
tial of the medial collateral ligament, a ligament
which is responsive to medical therapy (Murray
et al. 2006; Murrayet al. 2007). Cell proliferation
responses were shown to be similar between
the two ligaments. Therefore, the authors sug-
gested that the differences must be in the bio-
logic and mechanical environments. Similar to
the ACL, CrCL healing occurs in a unique envi-
ronment, intra-articularly but extrasynovial,
due to its being completely covered by a syn-
ovial membrane. When the CrCL is damaged,
the hematoma that usually forms a scaffold for
healing is dissipated in the joint. Also, when