3 BIOMECHANICS

3.1 Three-dimensional anatomy of the rotator cuff

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Ariane Gerber, MD, Fraser Harrold, MD, Maria Apreleva, PhD, Jon JP Warner, MD

Investigation performed at Orthopedic Biomechanics Laboratory, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston , MA, USA

3.1.1 Introduction

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The biomechanical relevance of the rotator cuff during active motion of the shoulder joint has been studied in several experimental (in vivo or cadaveric) and analytic models. In vivo studies have included the analysis of movement after palsy or nerve block.1 They have also estimated muscle force by electromyography2 and and evaluate pathologic kinematics of the shoulder with radiographic studies.3 Cadaveric studies have allowed to place known forces on the the motor units of the shoulder, while measuring the resulting movement of the joint are essentially kinematic.4-10 The effects of structural changes like pathologies or reconstructive procedures have been predicted in analytic models based on known anatomical and geometrical properties of the shoulder joint.11,12 Whereas the biomechanics of the normal rotator cuff has been extensively studied, they are only few information available in the literature describing the biomechanical effect of tendon transfer procedures around the shoulder.13.

The purpose of the present study was to define an experimental model able to describe the three-dimensional anatomy of the rotator cuff.

3.1.2 Material and Methods

3.1.2.1 Specimen preparation

For the purpose of this study 3 fresh human torsi (2 males, 1 female) including upper extremities, trunk and pelvis were available. One specimen which had a bilateral posterior dislocation of the shoulder was excluded. Therefore 4 hemi-torsi were available for analysis. Each specimen was thawed at room temperature for 24 hours prior to testing.

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For testing all specimens were fixed rigidely in the lateral decubitus. This allowed full access to the arm as well as the anterior and posterior sides of the shoulder girdle. All the skin was removed to expose the complete latissimus dorsi muscle, the pectoralis major and the serratus muscles as well as the muscles of the arm.

The deltoideus was completely resected to expose the underlying rotator cuff. Then the preparation was started with the supraspinatus muscle-tendon unit. The upper and lower edges of the muscle were identified. Dissection was carried on laterally up to the tendinous insertion side at the proximal humerus and the tendon was dissected from the underlying capsule. So the footprint of the tendon could be visualized at the greater tuberosity. The most anterolateral point of the tendon insertion was marked with a suture anchor. Using the same technique the most posterolateral point of the tendinous insertion was defined. At its muscular origine the most superomedial and inferomedial points were marked with ailet screws. Between insertion and origine 2 points along each the upper and lower edge of the muscle were marked with ailet screws. Now long braided number 2 sutures were fixed to the anterior and posterior edge of the tendon using a Bunnell stich configuration over a distance of approximatly 4 cm. At this level the muscle was cut and removed. Each pair of sutures was passed through the corresponding ailets at the lower and upper edges of the muscle. To tension the sutures and to simulating the three dimensional arrangement of the original muscle, several clamps were used.

Based on the same principle, both the infraspinatus, the teres minor, the upper and lower subscapularis, the sternal and clavuicular heads of the pectoralis major and the teres major were prepared.

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After dissection the scapula was fixed rigidely to the thorax with an external fixator. The arm was fixed in neutral rotation and an image amplificator was used to ensure that the glenohumeral joint was centered.

Fig 1a: Experimental set-up showing positionning of the specimen and the arm before data collection

Fig1b: Superior view of th especimen. Plexiglas cubes were used to create multiple coordinate systems.

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Fig 1c: The scapula was rigidly fixed to the thorax with an external fixator.

A Microscribe 3D-X digitizer (Immersion Corp., San Jose, CA) was used to register the three-dimensional anatomy of the joint and the above described muscles. The device was fixed rigidly to the custom built jig which was used to stabilize the specimens (Fig.2). Data collection was performed for four different positions of the arm:

  1. Neutral Position
  2. 45 degrees external rotation with the arm at the side
  3. 90 degrees of abduction with 90 degrees of external rotation
  4. Lift-off position with the elbow flexed at 90 degrees and the hand lying on the back at the level of L3.

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The position of the scapula on the thorax was not changed when the arm was moved from one to the other position of the glenohumeral joint.

Figure 2: A Microscribe 3D-X digitizer (Immersion Corp., San Jose, CA) was used to register the three-dimensional anatomy of the joint and the above described muscles. The device was fixed rigidly to the custom built jig which was used to stabilize the specimens.

3.1.2.2  Data collection

The Microscribe 3D-X system allows measurements to be made with the manufacturer’s reported accuracy of 0.23 mm. Dissections and all tasks involved in data collection were completed by the same operator (AG).

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The 3D-X digitizer was used to locate the insertion points, the leading edges and the origine points of following muscle-tendon units.

  1. Supraspinatus (SS)
  2. Infraspinatus (IS)
  3. Teres minor (TMi)
  4. Lower Subscapualris (SUS(L))
  5. Upper Subscapularis (SUS(U))
  6. Complete Subscapularis (SUS(T))
  7. Teres major (TMa)
  8. Sternal head of the pectoralis major (Pec(ster))
  9. Clavicular head of the pectoralis major (Pec(clav))

After that, the shoulders were separated so that the skeletal anatomy of the humerus and the scapula could be digitized.

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Multiple coordinate systems were used in this study: one to register the arrangment of the muscle-tendon units, one to register humeral anatomy, another to register scapular anatomy and a last one to combine all of the above data together. For registration of these multiple coordinate systems, small (2cm x 2cm x 2cm) registration blocks were manufactured from Plexiglas and rigidly attached to the humeral shaft and to the scapula. (Fig 1b). Three non-coplanar sides of the block were digitized at the beginning of each test and a local block coordinate system was built to serve as a cross-reference between different coordinate systems. The insertion site data, originally reported in the MicroScribe3DX™ coordinate system, were then transformed to a local block coordinate system.

3.1.2.3 Computer modelling and calculation

The collected data for each humerus were imported into Rhinoceros NURBS modeling software (McNeal and Assoc., Seattle, WA) and three-dimensional models were constructed for each shoulder. Modelling and calculation were performed by two investigators (FH, MA). Parameters describing the morphological structures were estimated from 3-D position coordinates of a large number of data points, using a least-square procedure. Tendon insertions and muscles were represented as a planes or as a (curved) line. Muscle paths were determined by a geometrical form of the bony contour around which the muscle was wrapped (Fig.3a and 3b). Hence force vectors could be calculated.

Figure 3a. Example of a model. Left shoulder from the anterior view. Humeral head(1), humeral shaft(2), clavicular head of the pectoralis major(3), sternal head of the pectoralis major(4), teres major(5).

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Figure 3a. Superior view. Humeral head(1), glenoid(2), clavicular head of the pectoralis major(3), sternal head of the pectoralis major(4), teres major(5).

To describe the force vector of each muscle, a transverse plane and a coronal plane perpendicular to the glenoid and to each other were used as references. α was defined as the angle between the constructed vectors and the coronal plane. The angle became negative when the vector was oriented backwards relative to the coronal plane. β was defined as the angle between the constructed vectors and the transverse plane. This angle was negative when the vector was oriented downwards relative to the transverse plane.(Fig.4)

Figure 4: Definition of the reference system to calculate the angles α and β

3.1.3 Results

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Tables I-IV gives a overview of the vector orientation of all muscles of the rotator cuff as well as the teres major and the pectoralis major.

TABLE I: Vector orientation with the arm in position 1

SS

(degrees)

IS

(degrees)

SUS (U)

(degrees)

SUS (L)

(degrees)

SUS (T)

(degrees)

Tmi

(degrees)

Tma

(degrees)

PEC (clav)

(degrees)

PEC (ster)

(degrees)

SP1

β

11

11

-4

-9

-7

-15

-3

18

66

α

2

-23

-6

-39

-23

-20

-21

60

15

SP2

β

5

13

-5

-5

-5

-13

0

7

39

α

-1

-17

-7

-33

-22

-30

-22

57

21

SP3

β

1

8

-7

-9

-10

-18

-6

33

72

α

3

-19

-2

-25

-16

-29

-18

49

3

SP4

β

16

20

6

3

5

-16

-1

38

72

α

-7

-25

-22

-44

-38

-40

-36

40

12

Mean (±SD)

β

8±6

13±4

-3±5

-5±5

-4±6

-16±2

-3±2

24±12

62±14

α

-1±4

-21±3

-9±8

-35±7

-25±8

-30±7

-24±7

52±8

13±6

TABLE II: Vector orientation with the arm in position 2

SS

(degrees)

IS

(degrees)

SUS (U)

(degrees)

SUS (L)

(degrees)

SUS (T)

(degrees)

Tmi

(degrees)

Tma

(degrees)

PEC (clav)

(degrees)

PEC (ster)

(degrees)

SP1

β

17

10

-2

-4

4

-6

-16

18

64

α

-2

-17

-9

-33

-19

-21

-22

56

16

SP2

β

12

19

-1

5

12

7

-33

8

57

α

2

-17

-2

-39

-5

-23

-14

55

29

SP3

β

5

14

-3

-11

3

-13

-14

36

69

α

4

-21

-5

-23

-25

-13

-20

44

9

SP4

β

23

26

3

3

16

2

21

21

63

α

-3

-22

-18

-42

-40

-34

 -19

 52

18

Mean (±SD)

β

14±7

17±6

-1±2

-2±6

9±6

-2±7

-21±8

21±12

63±5

α

0±3

-19±2

-8±6

-34±7

-22±13

-23±8

-19±3

52±4

18±7

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TABLE III: Vector orientation with the arm in position 3

SS

(degrees)

IS

(degrees)

SUS (U)

(degrees)

SUS (L)

(degrees)

SUS (T)

(degrees)

Tmi

(degrees)

Tma

(degrees)

PEC (clav)

(degrees)

PEC (ster)

(degrees)

SP1

β

7

11

-7

-6

3

-6

-10

9

50

α

3

-14

-5

-35

-20

-23

-18

73

40

SP2

β

-1

13

-1

0

3

-3

-1

9

61

α

8

-21

-12

-35

-25

-25

-29

63

24

SP3

β

4

-1

-14

-14

-22

-11

-19

37

73

α

-7

-13

1

-24

-18

-17

-26

51

11

SP4

β

2

21

5

8

9

7

10

18

61

α

-10

-28

-7

-40

-28

-28

-24

62

25

Mean (±SD)

β

1±3

11±8

-3±7

-2±8

-3±12

-3±7

-3±11

22±12

65±8

α

-3±7

-21±6

-6±4

-33±6

-24±4

-23±4

-26±4

59±8

20±10

TABLE IV: Vector orientation with the arm in position 4

SS

(degrees)

IS

(degrees)

SUS (U)

(degrees)

SUS (L)

(degrees)

SUS (T)

(degrees)

Tmi

(degrees)

Tma

(degrees)

PEC (clav)

(degrees)

PEC (ster)

(degrees)

SP1

β

13

14

-6

-7

-7

5

-10

23

53

α

2

-19

-9

-35

-28

-25

-28

13

-13

SP2

β

17

14

0

-1

1

16

-1

32

56

α

6

-15

-12

-40

-31

-29

-33

16

-9

SP3

β

3

6

-9

-9

-12

9

-1

43

57

α

-3

-30

-7

-24

-17

-27

-29

18

-9

SP4

β

9

27

-5

3

0

5

4

33

55

α

-9

-38

-8

-40

-30

-54

30

15

10

Mean (±SD)

β

11±5

15±8

-5±3

-3±5

-4±5

9±4

-2±5

33±7

55±1

α

-1±6

-25±9

-9±2

-34±7

-26±6

-34±12

-15±26

15±2

-5±9

3.1.4 Discussion

The result of this study gives a set of parameters for each cadaver, describing very precisely the geometry of the selected muscles of the shoulder. The experimental model and the vector analysis for four shoulder girdles presented here create the basis for the next chapter, where vectors of tendon transfers for subscapularis reconstruction are described and compared to the vector of the original subscapularis.

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Furthermore the collected data will be used in the future to develop an analytic shoulder model which eventually may help describe shoulder kinematic.

3.1.5 References

1. Gerber C, Vinh T, Hertel R, Hess C. Latissimus dorsi transfer for the treatment of massive tears of the rotator cuff. A preliminary report. Clin Orthop 1988 ;232:51 -61.

2. Comtet JJ, Herzberg G, Naasan IA. Biomechanical basis of transfers for shoulder paralysis. Hand Clin 1989 ;5:1 -14.

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3. Paletta GA, Jr., Warner JJ, Warren RF, Deutsch A, Altchek DW. Shoulder kinematics with two-plane x-ray evaluation in patients with anterior instability or rotator cuff tearing. J Shoulder Elbow Surg 1997 ;6:516 -27.

4. Mollier S. Über die Statik und Mechanik des menschlichen Schultergürtelsunter normalenund pathologischen Verhältnisse. Festschrift für C.v.Kupfer, Jena, 1899.

5. Thompson WO, Debski RE, Boardman ND, 3rd, Taskiran E, Warner JJ, Fu FH, Woo SL. A biomechanical analysis of rotator cuff deficiency in a cadaveric model. Am J Sports Med 1996 ;24:286 -92.

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6. van der Helm FC. Analysis of the kinematic and dynamic behavior of the shoulder mechanism. J Biomech 1994 ;27:527 -50.

7. McMahon PJ, Debski RE, Thompson WO, Warner JJ, Fu FH, Woo SL. Shoulder muscle forces and tendon excursions during glenohumeral abduction in the scapular plane. J Shoulder Elbow Surg 1995 ;4:199 -208.

8. An KN. Muscle force and its role in joint dynamic stability. Clin Orthop 2002 ;403 Suppl:S37-42.

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9. Mura N, O'Driscoll SW, Zobitz ME, Heers G, Jenkyn TR, Chou SM, Halder AM, An KN. The effect of infraspinatus disruption on glenohumeral torque and superior migration of the humeral head: a biomechanical study. J Shoulder Elbow Surg 2003 ;12:179 -84.

10. Halder AM, O'Driscoll SW, Heers G, Mura N, Zobitz ME, An KN, Kreusch-Brinker R. Biomechanical comparison of effects of supraspinatus tendon detachments, tendon defects, and muscle retractions. J Bone Joint Surg Am 2002 ;84:780 -5.

11. van der Helm FC. A finite element musculoskeletal model of the shoulder mechanism. J Biomech 1994 ;27:551 -69.

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12. An KN, Browne AO, Korinek S, Tanaka S, Morrey BF. Three-dimensional kinematics of glenohumeral elevation. J Orthop Res 1991 ;9:143 -9.

13. Magermans DJ, Chadwick EKJ, Veeger HEJ, Rozing PM, van der Helm FCT. Effectiveness of tendon transfers for massive rotator cuff tears: a simulation study. Clinical Biomechanics 2004 ;19:116 -22.

3.2 Tendon transfer procedures for irreparable subscapularis tears. A three-dimensional vector analysis

Ariane Gerber, MD, Fraser Harrold, MD, Maria Apreleva, MD, Jon JP Warner, MD

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Investigation performed at Orthopedic Biomechanics Laboratory, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston , MA, USA

3.2.1 Introduction

In tendon transfer surgery the matching of force vector orientation between the transferred and the dysfunctional muscle is a difficult task. The muscles available for transfer of a given dysfunctional musculotendinous unit are limited and their anatomical arrangement is usually very different from the muscle they should replace. Furthermore the moment arm of a transferred muscle may change and become less favourable because the position of the limb is changing.

Models describing the biomechanical effects of tendon transfer procedures around the shoulder in a comprehensive way are not yet avaible.1

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The geometrical model presented in chapter 3.1 turned out to be a suitable way to assess vector orientation of the normal rotator cuff muscles.

Based on the same methodology, it was the purpose of this study to determine the vectors of several tendon transfers commonly used for subscapularis reconstruction and to compare them with the vector of the original subscapularis.

3.2.2 Material and Methods

3.2.2.1 Specimen preparation

For the present study the specimens described in chapter 3.1 were used. Dissection and preparation of the muscles were performed in a same manner as described there.

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The following transfers were considered:

  1. PM-1: Transfer of the complete pectoralis major muscle (clavicular and sternal head) to the lesser tuberosity according to Wirth and Rockwood2
  2. PM-2: Transfer of the complete pectoralis major (clavicular and sternal head) rerouted underneath the conjoined tendon to the lesser tuberosity according to Resch3
  3. PM-3: Transfer of the sternal head of the pectoralis major rerouted underneath the clavicular head to the greater tuberosity according to Warner4
  4. TM-sPM: Combined tansfer of the teres major to the lower part of the lesser tuberosity of the sternal head of the pectoralis major, rerouted underneath the clavicular head to the upper part of the lesser tuberosity.5

For each muscle the origine and the inserting tendon were prepared and the muscle bellies replaced by sutures as described in chapter 3.1. The three-dimensional arrangement of each unit was digitized in neutral position of the arm for all specimens.

3.2.2.2 Data collection, modelling and calculation

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Using the experimental set-up described above, the transferred muscle-tendon units were digitized. The data were imported into into Rhinoceros NURBS modeling software (McNeal and Assoc., Seattle, WA) and three-dimensional models were constructed. To describe the orientation of the vector of each muscle a transverse plane and a coronal plane perpendicular to the glenoid and to each other were used as references. (Fig.1)

Again α was defined as the angle between the constructed vectors and the coronal plane. The angle was negative when the vector was oriented backwards relative to the coronal plane.

The angle β was defined as the angle between the constructed vectors and the transverse plane. This angle was negative when the vector was oriented downwards relative to the transverse plane.

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Figure 1: Two planes of reference were used to defined the vector orientation.

3.2.3 Results

3.2.3.1 Pectoralis major transfer according to Wirth and Rockwood (PM-I)

Table I is showing the vectors of the complete subscapularis muscle and the pectoralis major muscle-tendon unit transferred to the lesser tuberosity.(Fig.2)

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TABLE I : Comparision of the subscapularis vector and PM-I transfer vector

SUS (Tot)

(degrees)

PEC (Tot)

(degrees)

P Value°

SP1

β

-7

70

α

-23

-10

SP2

β

-5

73

α

-22

-3

SP3

β

-10

67

α

-16

-14

SP4

β

5

71

α

-38

-6

Mean ±SD

(degrees)

β

-4±6

70±2

< 0.0001

α

-25±9

-8±4

0.07

°Using the Student t-test for correlated groups at a significance level of p<0.05

The pectoralis major transfered in a conventional way to the lesser tuberosity is oriented anteriorly relative to the coronal plane. The mean difference between the transfer and the subscapularis was approximatively 65 degrees, which was highly significant (p<0.0001). Relative to the transverse plane both vectors were similar.

3.2.3.2 Pectoralis major transfer by Warner (PM-II)

Table II is showing the vectors of the sternal head of the pectoralis major rerouted underneath the clavicular head and attached to the greater tuberosity.(Fig.3)

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Figure 3: Anterior view of a right shoulder after PM-II tansfer. The sternal head (1) has been rerouted underneath the clavicular head and attached to the greater tuberosity.

TABLE II : Comparision of the subscapularis vector and the PM–II transfer vector

SUS (Tot)

(degrees)

PEC (Ster)

(degrees)

P Value°

SP1

β

-7

66

α

-23

-2

SP2

β

-5

71

α

-22

-6

SP3

β

-10

52

α

-16

-6

SP4

β

5

77

α

-38

-1

Mean±SD

(degrees)

β

-4±6

67±10

0.0002

α

-25±9

-4±3

0.04

°Using the Student t-test for correlated groups at a significance level of p<0.05

Rerouting the sternal part of the pectoralis major underneath the clavicular head significantly improved the orientation of the transfer compared to the original pectoralis major transfer (p=0.04). However the vector of the transfered unit remained significantly different compared to the vector of the subscapularis relative to both planes (p=0.0002 and p=0.04).

3.2.3.3 Pectoralis major transfer by Resch (PM-III)

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Table III is showing the vectors of the complete pectoralis major rerouted underneath the conjoined tendon and attached to the lesser tuberosity.

TABLE III: Comparision of the subscapularis vector and the PM-III vector

SUS (Tot)

(degrees)

PEC (Tot)

(degrees)

P Value°

SP1

β

-7

40

α

-23

-7

SP2

β

-5

44

α

-22

-9

SP3

β

-10

34

α

-16

-12

SP4

β

5

63

α

-38

-3

Mean±SD

(degrees)

β

-4±6

45±13

0.0005

α

-25±9

-8±4

0.078

°Using the Student t-test for correlated groups at a significance level of p<0.05

The data showed that with this transfer the vector relative to the coronal plane could be improved from an average of 70 degrees (conventional transfer) to 45 degrees(rerouted transfer). This difference was statistically significant (p=0.02). However comparision between the subscapularis vector and the vector of the transfer still remained different (p=0.0005)

3.2.3.4 Combined teres major-split pectoralis major transfer (TM-sPM transfer)

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Table IV is showing the vector of the teres major transfer and comparing it with the vector of the lower subscapularis. The table shows also the comparision between the vector of the rerouted sternal head of the pectoralis major after transfer to the lesser tuberosity and the vector of the upper subscapularis.(Fig.4)

Figure 4: Anterior view of a right shoulder after TM-PM tansfer. The teres major(1) has been transferred to the lower part of the lesser tuberosity, whereas the sternal head of the pectoralis major(2) has been rerouted underneath its clavicular head(3) and attached to the superior part of the lesser tuberosity. Conjoined tendon (4)

TABLE IV : Comparision of the subscapualris vectors and the TM-PM transfer vectors

SUS(L)

(degrees)

TMa

(degrees)

P Value°

SUS(U)

(degrees)

PM(Ste)

(degrees)

P Value°

SP1

β

-9

-7

-4

68

α

-39

-34

-6

-22

SP2

β

-5

-9

-5

66

α

-33

-42

-7

-13

SP3

β

-9.45

-17

-7

63

α

-25

-38

-2

26

SP4

β

3

-13

6

76

α

-44

-54

-22

0

Mean ±SD

(degrees)

β

-5±6

-12±5

0.17

-2±6

68±5

<0.0001

α

-35±8

-42±9

0.21

-9±9

-15±12

0.59

°Using the Student t-test for paired groups with a significance level set at p<0.05

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In this model we could not find a difference between the vector orientation of the teres major and the lower part of the subscapularis (p=0.17, p=0.21). Comparing the reconstruction of the upper subscapularis with split pectoralis major, the difference of the vector orientation was significant relative to the coronal plane (p<0.0001). Relative to the transverse plane the vector orientation of the upper subscapularis and the split pectoralis major were similar (p=0.59).

3.2.4 Discussion

The present study is the first description of the three dimensional geometry of tendon transfer procedures around the shoulder. In their study Magermans et al. used the finite element model described by van der Helm to anlayze different configurations of the latissimus dorsi and the teres major transfers for reconstruction of the posterior rotator cuff 6 In their analysis the authors did not assess geometry of the tendon transfers experimentally.

With this experimental model it was possible to quantitatively describe the influence of rerouting procedures when performing a pectoralis major transfer. The most effective way to change the anterior orientation of the pectoralis major vector was to reroute it underneath the conjoined tendon. However none of the proposed techniques was able to restore the vector of the subscapularis relative to the coronal plane.

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The data furthermore demonstrated that the pectoralis major transfers and the subscapularis have a similar orientation relative to the transverse plane. As the subscapularis is, the pectoralis major transfers described in this work all were oriented downwards.

In chapter 2.2 it was demonstrated, that anatomically speaking the teres major is a valuable transfer for reconstruction of the lower subscapularis. With the present analysis it could be shown that this muscle exactly replicates the three-dimensional geometry of the axillary subscapularis.

A weakness of the present analysis is the small number of specimens. To create an analytic model based on these data further measurements will be required.

3.2.5 References

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1. Magermans DJ, Chadwick EKJ, Veeger HEJ, Rozing PM, van der Helm FCT. Effectiveness of tendon transfers for massive rotator cuff tears: a simulation study. Clinical Biomechanics 2004 ;19:116 -22.

2. Wirth MA, Rockwood CA, Jr. Operative treatment of irreparable rupture of the subscapularis. J Bone Joint Surg Am 1997 ;79:722 -31.

3. Resch H, Povacz P, Ritter E, Matschi W. Transfer of the pectoralis major muscle for the treatment of irreparable rupture of the subscapularis tendon. J Bone Joint Surg Am 2000 ;82:372 -82.

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