An apparatus called “tensioner” allowing application of tension to the infraspinatus muscle-tendon-unit without necessitating limb immobilization was designed:
Tensioner (Figure ):
Figure 1 Tensioning device with holes for locking screws through the baseplate(A). Through axial rotation on the steel wheel (E) the threaded rod (C) exits the opposite side of the body (B) and thereby elongates the muscle tendon unit, which is fixed to the end (more ...)
The tensioning device (“tensioner”) comprised a base plate (A) for fixation, which was welded to a longitudinal body (B). The body of the tensioner housed a threaded rod (C, threads hidden in the body), which was driven out of the body (B) by turning a rotating axis (D) which entered the body (B) on the opposite side of the rod. The rotating axis was made of Nitinol® and had a length of 155
mm. At the end of the rotating axis, a steel wheel of 20
mm diameter was fixed to allow lengthening of the threaded rod. Turning of the rotating axis by 360° results in an extrusion of the rod by 1
mm. A clicking mechanism gives a small resistance after every full turn (1
mm, 360°) of the axis and inhibits unwanted passive re-rotation of the rod. The rod displaces rotating clockwise and that the click mechanism prevents counter clockwise rotations. This axis (D) is connected to the body axis (B) with a universal (or Cardan) joint to allow angulations of axis (D) relative to the body axis (B). The steel wheel (E) mounted on rotating axis D is external to the sheep’s body. The Cardan joint is protected from spontaneous scarring and tangling with adjacent soft-tissue with a silicon tube. The apparatus is fixed on the scapular spine of the sheep with locking screws, which prevent angular shifting of the baseplate.
Two different tensioners were manufactured and implanted during experiments. Tensioner No.1 was developed in collaboration with the AO, Development Center (ADI, Davos, Switzerland). For fixation of the tensioner in-vivo experiments, device No. 1 consisted of a base plate welded to the body of the tensioner. The frame was then fixed to the scapular spine using a maximum of four screws in the fixation plate (Figure ).
Tensioner No.1 mounted on the scapular spine of the sheeps’ shoulder with 4 holes for fixed angle locking screws and a bent threaded rod ‘end’.
The threaded rod allowed expanding the device by 35
mm. The distal part of the threaded rod was bent twice to bring its end in line with the muscle tendon. The end of the 3
mm diameter rod was equipped with an eyelet (3.5
mm radius; 1
mm thick) to secure the sutures holding the tendon to it during the “in-vivo-experiments”. Because of cut outs of four baseplates out of the scapular spine with tensioner No. 1, it was changed to the technically improved tensioner No. 2 over the course of the experiment. This tensioner No. 2 was performed in collaboration with Synthes (Synthes GMBH, Bettlach, Switzerland). The second generation tensioner consisted of a longer base plate with eight screw holes for fixation of the base plate to the scapular spine (Figure ).
Tensioner No. 2 with expansion of the baseplate and 8 possible screw placements.
The same diameter, thickness and materials were used for the base plate, body, threaded rod, pin and loop of the tensioner No. 2. (produced by Synthes GmbH, Solothurn, Switzerland). To reach more stability the multiaxial joint was made of stainless steel. Both, the Nitinol®-rod and the steel wheel had a higher diameter of 2
mm, and 32
In vitro experiments:
Expansion force generation
Axial forces developed actively with both tensioners by extruding the threaded rod from the body, were measured using an Instron Universal Testing Machine 4202 (Instron Limited, High Wycombe, HP12 3SY, England) with a 5kN load cell, mounted to the cross-bar. The sample was fixed to the lower table of the machine, using a vice holding the body of the scapula. The mean maximal axial force that can be developed by hand with each tensioner was tested by three different examiners (EF, DM, and MZ).
Three tensioner units of versions No. 1 and No. 2 were tested with cycling loading experiments. The tensioners were fixed on the scapular spine with 4 screws in tensioner No. 1 and six screws in tensioner No. 2, respectively. They were mounted on an Instron Universal Testing Machine 4202 (Instron Limited, High Wycombe, HP12 3SY, England) with the adjustable fixation device, which allowed adjustment of direction of pull. The direction of load application corresponded to the axis of the infraspinatus muscle in vivo (Figure ).
The setup of the loading direction made up in line with the sheeps’ infraspinatus muscle.
Each tensioner was tested individually. To test the most vulnerable configuration, the tensioner was maximally extended to gain the longest lever arm. Cyclic loading was performed to simulate the in vivo conditions best, similar to prior studies [10
]. In both tensioners, a constant base load of 20
N was applied. For both tensioners, one hundred cycles of 10
N (lower limit) to 100
N (upper limit) were applied with a crosshead speed of 20
mm per minute and showed a sinus load curve. Further continuous increase of the upper load limit was applied using the same setup and cycles until failure occurred. The ultimate tensile strength and the modes of failure were recorded for each tensioner.
In vivo experiments:
In-vivo experiments were performed with 12 white alpine sheep. The mean age of the sheep at the beginning of the experiments was 15.5
months, with a range from 14 to 17
months. The mean weight at the beginning of the experiment was 45.2
kg. All experiments were conducted according to the Swiss law of animal welfare and use for animal experiments (permission number ZH nr. 193/2004).
The surgical approach was performed using a curved incision over the lateral aspect of the acromion [7
] under general anaesthesia [23
]. According to the previous study [7
], the infraspinatus was released using an osteotomy of the greater tuberosity. Two Fibre wire No. 5 sutures (Arthrex, Naples, FL) were passed in a figure-of-eight configuration through tendon and bone. The sutures and the greater tuberosity were wrapped into a 5
cm long silicone rubber tube (Silicone Penrose drain tube, 12
mm, Fortune medical instrument corp. Taipei, Taiwan). The tube was closed with non-resorbable suture at its end to prevent spontaneous healing. After a retraction and degeneration time of 40
weeks, the tensioner was implanted using the same approach. The locking-fixation-plate was mounted on the scapular spine, at the side of the supraspinatus muscle, with 4 locking plate screws in tensioner No.1 and 7 locking plate screws in tensioner No. 2. Tensioner No. 1 was primary implanted in 10 and tensioner No. 2 in two sheep. To prevent infection, the rotational axis was passed through a 150
mm subcutaneous tunnel before exiting through the skin. The sutures were knotted with repeated surgical knots on the eyelet of the treaded expanding rod and a basic tension of 20
N was applied.
By turning the steel wheel 360° once a day the tensioner pulled the tendon axially by a distance of 1
mm/day towards its original insertion.
To document the elongation of the musculotendinous unit, computer tomograms were performed after implantation and every 14
days under general anesthesia. The position of the tensioner and the greater tuberosity were measured in relation to the midglenoidal plane.
Subsequent to the elongation protocol, the tensioner was removed and the greater tuberosity was reattached as near as possible to its original insertion using 3.5
mm cortical bone screws and the previously prepared sutures. According to a previous study [7
], the fixation was augmented by two Fiber Wire No. 2 sutures (Arthrex, Naples, FL). Postoperatively, rehabilitation included prevention from lying down by using a loose suspension net [7
]. To avoid full weight bearing, a ball was attached to the sheep’s’ claw for six weeks postoperatively.