A biomechanical and histological comparison of the suture bridge and conventional double-row techniques of the repair of full-thickness rotator cuff tears in a rabbit model

Rotator cuff repair has a high rate of failure. Factors affecting bone-tendon healing
after rotator cuff repair include suture strength 6], bone-tendon contact area and pressure 7], fretting between the tendon and bone 8], fatty degeneration of the rotator cuff, the size of the rotator cuff tear 9], the quality of humeral bone 9], 10], suture type 11], and postoperative rehabilitation 12]. Repair techniques may be an important factor affecting the success rate considerably
13]. Numerous biomechanical studies have compared single-row (SR) fixation to DR fixation.
Most of these studies have showed that DR repairs reconstruct the anatomic footprint
of the rotator cuff significantly better than SR repairs, and DR repairs have less
gap formation, better tensile strength, and a lower failure load than SR repairs 14]–21]. More and more doctors are selecting anatomic footprint reconstruction, and the suture
bridge has recently been described as a new technique because of the increase of the
bone-tendon contact area and pressure 22]–25]. The contact area of double-row and suture bridge fixation in fresh-frozen human
shoulders has been tested previously 2]. Results showed that the mean pressurized contact area between the tendon and insertion
was significantly greater for the 4-suture bridge technique (124.2 +/? 16.3 mm², 77.6 % footprint) than the double-row technique (63.3 +/? 28.5 mm², 39.6 % footprint). Their study showed that the SB technique may facilitate healing
more effectively than the DR technique. However, a few studies have shown direct evidence
that SB fixation has better healing and biomechanical properties in vivo. Clinical
outcomes of SB technique have shown mixed results when compared to DR techniques 5], 26]–28].

Previous studies using a rabbit subscapularis model have shown that at time zero the
SB technique had 38.5 % higher ultimate load than the DR group 29]. A cadaveric study showed the SB technique had a 48 % higher ultimate load than DR
repair at time zero 3]. The current study showed that the SB group had a higher ultimate load (134.59?±?17.69)
than the DR group (103.83?±?6.62) after 8 weeks although both repair groups had significantly
less ultimate load than the control group (199.25?±?14.81) (P?0.01). Miyahara et al. studied the mechanical strength of the supraspinatus after
reinsertion in dogs and reported that the load at failure was 29.8 % that of controls
at two weeks, 62.5 % at six weeks, and 82.5 % at 24 weeks 30], compared to controls 30]. Gerber et al. found that the ultimate failure strength of the reattached infraspinatus
in sheep was 30 % at six weeks, 52 % at three months, and 81 % at six months 31]. Their studies showed that solid healing of bone-tendon interface needed more time.
This was consistent with the present findings and could explain why the SB group had
better healing than the DR group but significantly less load to fail than control
group at the 8^th week.

Recent reports indicate that the retear rate after surgery remains remarkably high,
ranging from 30 % to 94 % 13], 32], 33]. Some clinical follow-up evaluations have reported that the retear rate after a DR
repair for large and massive tears ranged from 40 % to 64 % 34], 35]. Mihata et al. compared the retear rates of the DR and SB technique using MRI after
arthroscopic rotator cuff repair 26]. The retear rates were 26.1 %, and 4.7 %, respectively, for the DR and SB groups.
In the subcategory of large and massive rotator cuff tears, the retear rate in the
SB group (3 of 40 shoulders, 7.5 %) was significantly lower than in the DR group (5
of 12 shoulders, 41.7 %, P?0.01). The low retear rate indicated that the SB technique could lead to a more
robust healing than the DR technique, especially when for large rotator cuff tears.
This may have been caused by differences in the healing patterns. At the 2^nd week postoperatively, the collagen fibers in SB group were found to be more compact
than in the DR group at the bone-tendon junction, and the collagen fibers were arranged
irregularly in both groups (Fig. 5a and b). At the 4^th week postoperatively, the collagen fibers in SB group extended into the cancellous
bone in columns, but those in DR group were distributed indiscriminately with cartilage
cells. During this period, the cartilage cells in the SB group also grew in columns
containing with collagen fibers, but the morphology of the cartilage cells were immature
(Fig. 6a and b). At the 8^th week, the structure and morphology of collagen fibers and cartilage cells in SB group
were more mature and approximate to a normal structure compared with those in DR group
(Fig. 7a and b). Anatomic reduction of fracture could produce a direct healing. For this reason,
it was here proposed that the SB repair with more bone-tendon pressure could produce
direct healing pattern of bone-tendon interface. The difference in morphology of collagen
fibers and cartilage cells at the bone-tendon interface may have been caused by differences
in bone-tendon contact pressure.

Currently, there is no clinical evaluation standard including the postoperative retear
rate. Park et al. compared the clinical aspects of the DR repair and the SB technique
but failed to demonstrate clinical differences between the two techniques 5]. The reason for this may be that the form of clinical evaluation used here is not
comprehensive enough. It should include the retear rate. The current study provides
direct evidence showing the SB technique to be superior to DR.

There are two limitations to the current study. First, the small area of the rabbit’s
greater tuberosity prevented the use of suture anchors, as in human surgery. Similar
repairs were made by suturing through the bone. Transosseous suture fixation techniques
may produce certain effect to the experimental results. Second, follow-up time was
limited; the healing of a rotator cuff needs more time. Some improvements including
the repair device and a longer follow-up time should be completed in future studies.