Advances in the Canine Cranial Cruciate Ligament, 2nd edition

Cruciate Ligament Remodeling and Repair 25

(A)

(B)

Week 1

Week 7

Figure 3.2 Macrophage localization within the
reconstructed cranial cruciate ligament (CrCL).
Immunohistochemistry of the circulating M1
macrophages within a reconstructed rat CrCL at 1 week
(A) and 7 weeks (B) after transplantation. Similar to the
canine reconstructed CrCL, few macrophages localize to
the graft at week 1 after injury. As time progresses, more
cells infiltrate the ligament. Original magnification,×600.

(Figure 3.2). Neutrophils and macrophages,
derived from the bone marrow or synovial
membrane, accumulate within the outer graft
by day 4 (Kawamuraet al. 2005). Proliferating
cells and M1 and M2 macrophages continue to
progress into the inner tendon and are evident
by 14 days post-transplantation (Kawamura
et al. 2005). Few to no neutrophils or T lympho-
cytes localize to the inner graft (Kawamuraet al.
2005). Evidence suggests that the macrophages
differentiate into a fibroblast phenotype to
synthesize collagen (Vaage & Lindblad 1990).
During remodeling, the graft undergoes lig-
amentization, with characteristic restructuring
of the graft towards the properties of the intact
CrCL (Amiel et al. 1984; Amielet al. 1986;

Murrayet al. 2000; Marumoet al. 2005). Col-

lagen fibers regain organization, which resem-

bles the appearance of the intact CrCL. How-

ever, the initial loss of collagen crimp, size of

collagen fibrils, and parallel alignment is only

partially restored (Liuet al. 1995; Scheffleret al.

2008). Additionally, the graft remains mechan-

ically inferior when compared to the uninjured

tissue. Grafts initially pull from the osseous tun-

nel. Astudy in sheep showed that load to failure

of the reconstructed graft averaged 5% of the

normal ligament at 3 weeks (graft 37.8±17.8 N

versus intact 759.2±114.1 N), and typically

fails in the intra-articular portion (Melleret al.

2008). Graft load to failure improved to 15%

of the normal ligament by 6 weeks, and exhib-

ited a decline in grip-to-grip elongation (Meller

et al. 2008). By 12 and 24 weeks, the strength

of the graft had improved to 41% and 69% of

the normal ligament, respectively, but the graft

strength remained far less than the normal CrCL

at even 52 weeks after transplantation (Meller

et al. 2008).

Healing potential of the reconstructed

graft interface tissue

CrCL reconstruction requires healing of the

tendon graft and bone tunnels in the femur

and tibia. Repair of the CrCL proceeds via

the formation of a fibrovascular interface tis-

sue (Figure 3.3) between the graft and bone,

bone in-growth into the graft–bone interface,

and the progression of collagen fiber continuity

between tendon and bone to result in a reestab-

lished tendo-osseous junction (Rodeoet al. 1993;

Rodeoet al. 1999). After CrCL reconstruction,

the graft and original cancellous bone fills with

fibroblast-deposited fibrovascular interface tis-

sue (Figure 3.4). Inflammatory cells and type

III collagen initially accumulate in the inter-

face granulation tissue. A few neutrophils and

T lymphocytes are found within the granula-

tion tissue the first week after surgery (Kawa-

muraet al. 2005). After the neutrophil numbers

have decreased, M1 macrophages, followed by

M2 macrophages, appear. While the inflamma-

tory cells infiltrate the granulation tissue, new

bone also permeates the interface. Chondroid

cells then appear from the side of the bone

tunnel and degrade the granulation tissue and