The concept of non-anatomic reversed arthroplasty is becoming increasingly popular. The design medializes and stabilizes the center of rotation, and lowers the humerus relative to the acromion, and lengthens the deltoid muscle up to 18%. Such a surgically created global distraction of muscles is likely to affect nervous structures. When nerves are stretched up to 5–10%, axonal transport and nerve conduction starts to be impaired. At 8% of elongation, venous blood flow starts to diminish and at 15% all circulation in and out of the nerve is obstructed. [1] To understand nerve dynamics following reversed arthroplasty, we investigated nerve strain and excursion in a cadaver model.

In a formalin-embalmed female cadaver specimen, the brachial plexus en peripheral upper limb nerves were carefully dissected and injected with an iodine containing contrast medium. At the same time 1.2 mm-diameter leaded markers were implanted at topographically crucial via points for later enhanced recognition on CT reconstructions. After the first session of CT scanning a plastic replica of the Delta reversed shoulder prosthesis® was surgically placed followed by re-injection of the plexus with the same solution. The preoperative and the postoperative specimen were studied using a helical CT scan with a 0,5 mm slice increment. The Mimics® (Materialise NV, Belgium) software package was used for visualization and segmentation of CT images and 3D rendering of the brachial plexus and peripheral nerves.

After surgery, there was an average increase in nerve strain below physiologically relevant amplitudes. In a few local segments of the brachial plexus an increase in nerve strain exceeding 5–10 % was calculated. The largest increase in strain (up to 19%) was observed in a segment of the medial cord. These results suggest there might be a clinically relevant increase in nerve strain following reversed shoulder arthroplasty.

Physiological studies have revealed that the central nervous system controls groups of muscle fibers in a very efficient manner. Within a single skeletal muscle, the central nervous system independently controls individual muscle segments to produce a particular motor outcome. Mechanomyographic studies on the deltoid muscle have revealed that the deltoid muscle, commonly described as having three anatomical segments, is composed of at least seven functional muscle segments, which all have the potential to be at an important level independently coordinated by the central nervous system.[1] In this study we tried to anatomically describe and quantify these different functional segments within the deltoid muscle, based on the branching out pattern of the axillary nerve.

Forty-four deltoids of 22 embalmed adult cadavers, were analyzed. The axillary nerve was carefully dissected together with his anterior and posterior branch upon invasion into the muscle. According to the pattern of fiber distribution and their fascial embalmment, we then carefully splitted the deltoid muscle into different portions each being innervated by a major branch of the axillary nerve. The position and volume of each segment in relation to the whole muscle was derived.

In 3 cases the axillary nerve branched out in 8 major divisions. In 22 out of 44 cases (50%), the axillary nerve branched out in 7 principal parts. A branching out pattern of 6 major divisions occurred in 14 out of 44 cases. Finally we found a division in 5 major branches in 5 of the specimens. In general, both posterior and anterior peripheral segments seemed to have the largest volume. In nearly all (93%) cases, the central segments were smaller in weight and volume compared to the more peripheral segments.

Based on the innervation pattern of the deltoid muscle a segmentation in 5 up to 8 major segments seem to be found. This confirms from anatomical point of view earlier reports of functional differentiation within the deltoid muscle.

Biomechanical models have been successfully applied to screen potential risk factors for injuries and to plan and evaluate the effects of orthopedic surgical procedures.[1] These models have made apparent the feasibility and necessity for the generation of subject specific models that are aimed at custom clinical applications. In order to develop such models a methods needs to be developed that allows accurate geometrical visualization and reconstruction of position and characteristics of bone and soft tissues, including neurovascular structures.[2] In this study, we present our approach to obtain both bony as soft tissue features necessary for upper limb modeling from computer tomography alone. As a case study the techniques were applied in a non-anatomic shoulder reconstruction.

In order to determine the muscles of the shoulder girdle, ultrathin flexible metallic markers were sutured from origin to insertion according to the fiber directions in all muscles involved in shoulder movement on a total of ten different cadaver shoulders. The plexus brachialis and upper limb nerves were dissected and injected with a iodium contrast containing mixture. A Ct multi-slice image reconstruction was performed from occiput to the hip joint. The software package Mimics® (Materialise NV, Heverlee, Belgium) was used to segment and reconstruct the different anatomical models that included bone, muscle features, nerves and vascular structures. A clustering method algorithm, was used to filter interruptions of the different masks, scattering rustle and small irregularities due to the different contrasting markers used. Vascular tissue could be reconstructed and segmented as air filled structures. We were able to accurately reconstruct nerve tissue in an highly complex configuration such as the plexus brachialis.

Analysis of the representations showed that the different morphologic parameters were within the normal anatomical ranges and that our method is suitable to create complete anatomical models based on Ct-imaging alone.