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ORIGINAL ARTICLE

A Comparison Study Between Three Constrained Total Shoulder Replacements Designed for Proximal Humeral Sarcoma Limb Salvage Cases

 Vicatos George*, Hosking Keith **,Wessels Gregory *

* Mechanical Engineering Dpt, University of Cape Town , Rondebosch 7701, Cape Town , South Africa
**Groote Schuur and Vincent Pallotti hospitals, Cape Town , South Africa

Address for Correspondence:

Vicatos George
 
Mechanical Engineering Dpt, 
University of Cape Town, Rondebosch 7701, 
Cape Town, South Africa, 
Tel: +27216502492,
Email: george.vicatos@uct.ac.za

 

Abstract:

The Total Shoulder Replacement (TSR) in patients with Proximal Humeral Sarcoma (PHS) presents a challenge as standard prostheses rely on the muscles of the shoulder joint for stability. The loss of stability in PHS cases is due to the resection of both the rotator cuff and deltoid muscles and therefore the prosthesis requires built-in stability. This report discusses three designs, which are the Quad-Point TSR, the Hybrid-Screw TSR and the Central-Peg TSR. All three utilize the same constrained ball-in-socket articulating design, which links the glenoid and humeral components. Finite element analysis (FEA) and experimental testing were carried out on the ball-in-socket system. The ball-in-socket design was found to have tensile and moment strengths of 900N and 15Nm respectively and failure was noted when any part of the ultra high molecular weight polyethylene (UHMWPE) socket experienced plastic yielding. This link has a 90o range of mobility.  All three of the TSR designs incorporate modularity and suture holes in their respective humeral components, while both cement and bone screws have been used in the glenoid fixation.  The use of the coracoid process, as an extra fixation point for the glenoid component, is possible as the coracoid process is exposed due to the resection of the rotator cuff.

J.Orthopaedics 2007;4(3)e22

 Keywords:
Shoulder Sarcoma; Proximal Humeral Sarcoma; Constrained Shoulder; Reverse Shoulder Prosthesis.

Introduction:

Patients with Proximal Humeral Sarcoma (PHS) require a Total Shoulder Replacement (TSR) in order to salvage their upper limb. In most cases, the sarcoma affects all of the primary stabilizing muscles of the shoulder joint and as result built-in stability is a requirement in the prosthesis design. This principle conflicts with widely accepted designs, which attempt to simulate the anatomical function of the shoulder joint.  Further difficulties associated with the presence of cancer are the reduced availability of skin required for wound closure, poor bone stock due to chemotherapy and varying levels of bone resection 1.  

This work discusses three constrained total shoulder replacement designs having the following criteria:

  • Modularity of the components to accommodate for differing levels of humeral resection,

  • Producing a low prosthesis volume to ensure that enough skin is available for wound closure,

  • Increased glenoid fixation strength with both cement and screw fixation

  • A constrained link between the glenoid and the humeral components, which provides a high range of mobility and strength.

Background:

  Historical review

Neer reported the first series of prosthetic shoulder arthroplasty in 1955 2, and his designs were unconstrained. The prosthesis consisted of a press fit cobalt chrome humeral head and in principle was designed to recreate normal anatomy. Neer’s unconstrained anatomical prostheses have become the standard for patients with intact rotator cuff.  

For the treatment of more severe shoulder disorders, where for example, the rotator cuff is deficient, constrained TSRs were developed during the mid 1970s and early 1980s. Despite early favourable results, most of the systems have been abandoned because of the high incidence of complications.3,4,5,6.  The complications were due to loosening, instability and fracture of the components because of the combined compressive and shear forces leading to excessive stresses on the components and bone 7,8,9.   

  Operative Principles

A proximal humeral sarcoma can be detected as a growth just below the shoulder joint. The only way to assess the cell type that makes up the growth is to take a biopsy of the tissue. During the definitive operation, the skin immediately surrounding the biopsy incision is also removed, which reduces the amount of skin available to close the wound. To facilitate closure of the wound the implanted prosthesis must therefore have the lowest volume possible.  

When a sarcoma is resected the objective of the surgery is to perform a wide resection of the tumour containing a margin of health tissue/musculature. This results in the loss of the rotator cuff muscle group and often the axillary nerve, which supplies the deltoid muscle. Very few of the shoulders stabilizing muscles remain. The stability that these muscles once provided must now be built into the prosthesis. There are however, remaining segments of muscle, the extent of which largely depend on the size and location of the tumour. This remaining muscle should be attached to the prosthesis via suture holes, which will increase the stability and may restore part of the joints active function.  

In resecting the rotator cuff the surgeon can gain access to the coracoid process, which can be utilised for additional anchorage thereby enhancing the glenoid component fixation.  

 Market Review

The Bayley-Walker TSR is one of the only constrained total shoulder replacements still available on the market today. It is based on the Kessel design which resulted in a pain-free joint but was associated with a high incidence rate of glenoid-fixation loosening 10. The Bayley-Walker TSR is a reverse anatomy prosthesis which consists of a titanium/UHMWPE glenoid component with a Co-Cr-Mo alloy head 11 (Fig 1). It has been designed specifically for patients with difficult reconstruction problems, rotator cuff arthroplasty and disruption of the superior coraco-acromial arch 11. In addition, it has been used for treating bone tumours of the proximal humerus where a segmental humeral component is utilized, but it lacks the following features, which are required in prostheses suitable for patients with PHS.

·         Suture holes are not provided for fixing any remaining muscle.

·         Modularity has not been built into the humeral stem to accommodate different levels of resection

·         The use of cement in the fixation of the glenoid component is not possible.

The aim of the designs discussed in this paper is to address the above deficiencies.

 

 

 

 

 

 

 

 

 

 


Fig 1. The Bayley-Walker shoulder joint (a) and the implanted Bayley-Walker shoulder (b) 11. 

 Design and development

Biomaterial

The biocompatibility of Ti-6Al-4V, Co-Cr-Mo and UHMWPE is proven, and the wear couple between Co-Cr-Mo and UHMWPE is a standard, used for years.

Constrained Link

The link between the humeral fixation and the glenoid fixation must have some level of built-in stabilization otherwise the mass of the limb may excessively load the remaining muscle which has been sutured to the prosthesis and cause tearing of the tissue. The constrained link is achieved by using a ball-in-socket system, which only allows rotational motion. The ball-in-socket design provides the least volume, which is a paramount factor to ensure wound closure.  

To optimize the range of motion and the dislocation strength in both tension and bending of the ball-in-socket system, FEA and experimental testing was carried out using an ABAQUS linear analysis and a Zwick tensile tester, respectively. In the initial design the ball was press-fitted into a one-part UHMWPE socket, after which the coupled ball-and-socket was inserted into the housing (Fig 2).  

 

 

Fig 2. FEA simulation of the one-part socket design. Step1: start of analysis, Step 2: the ball is inserted into the cup, Step 3: The ball and cup are inserted into the housing, Step 4: The ball begins to dislocate from the housing, Step 5: The ball has been fully dislocated.  

The final ball-in-socket design (Fig 3) is based on the proposed design by ISIQU Orthopaedics and consists of a Co-Cr-Mo ball-and-stem articulating with a UHMWPE socket. The socket is split beyond the equatorial plane. This system is housed in the titanium body of the prosthesis and is locked together using a ring clip. The socket is prevented from rotating by a Woodruff key type peg.

 

Fig 3. Constrained link assembly.  

The housing acts as a retainer preventing the UHMWPE from expanding during the loading of the ball. The containment of the socket increases the retention force of the system by 300% 12. The limiting factor of this design is the minimum allowable entry diameter of the socket before the ball produces plastic deformation in the socket. Using a 20mm diameter ball the minimum allowable entry diameter is 19.4mm. The design will produce a maximum retention force of 400N at which point dislocation begins to occur. The low retention force was unacceptable and the design was modified and consisted of a split socket. The ball is inserted into the top half of the socket from below and the rest of the assembly process remains the same as for a one part socket.  Using this design it is possible to generating a retention force of 900N before plastic yielding of the UHMWPE occurred, using an entry diameter of 18mm in 20mm diameter socket. The point at which the UHMWPE experiences plastic yielding under the influence of a moment is 15Nm and the range of motion produced by the ball-in-socket is 90o (Fig 4).

 

 

 

 

 

 

Fig 4. Range of motion of the ball-in-socket constrained link design.

Discussion:

  Comparison of designs

Glenoid fixation and the orientation of the articulating ball differentiate the three designs. Table 1 lists and quantifies the factors that contribute to the performance of glenoid fixation.  The articulating ball can either be orientated anatomically (on the humeral component) or in reverse (on the glenoid component) as shown in Fig 5.

 

          (a)                                         (b)                                   (c)

Fig 5. Three constrained total shoulder replacement designs. (a) Hybrid-Screw TSR, a reverse anatomical design , (b) Central-Peg TSR, a reverse anatomical design  and (c) Quad-Point TSR, an anatomical design.  

Table 1: The design factors.

Factor

Desired

Quad-Point TSR

Hybrid-Screw TSR

Central-Peg TSR

Anatomical or reverse ball and socket

-

Anatomical

Reverse

Reverse

Centre of rotation from the glenoid face

min

18mm

24mm

24mm

Distance from humeral shaft axis to glenoid face

min

46mm

41.4mm

41.4mm

Range of Mobility (ROM)[1]

Elevation

160°

160°

160°

160°

Extension

60°

60°

60°

60°

External Rotation in

Abduction

60°

52°

52°

52°

Internal Rotation in

Abduction

60°

38°

38°

38°

Abduction[2]

140°

125°

125°

125°

Posterior Reach

S1

None

None

None

Glenoid Fixation Factors

Number of screws used in the glenoid fixation 

max

3

2

1

Number of attachment points using cement as fixation

max

1

3

3

Volume of Bone

Removed[3]

min

2.87cm3

 

4.67 cm3

5.16 cm3

Contact area of cement

on bone

max

 

9.2mm2

11.7 mm2

11.7 mm2

Contact area of cement

on prosthesis

max

5.2 mm2

6.4 mm2

6.4 mm2

Contact area of prosthesis on bone

max

14.1 mm2

17.7 mm2

18.7 mm2

 

   Glenoid fixation

Loosening of the glenoid component is a major cause of failure in unconstrained total shoulder arthroplasty 13. In the case of constrained devices, glenoid fixation is exposed to higher shear loads as all shear loading is transferred from the humerus to the glenoid fixation due to the devices’ inability to translate.  In the past, constrained devices failed when they were implanted in patients with intact rotator cuff and deltoid muscles 14.  This may be attributed to high loading and usage conditions of the limb. In the case of PHS patients the lack of muscle prevents much of the loading activity and the main function of the prosthesis is to salvage the limb and allow the patient to have the function of the forearm for activities such as writing, typing and personal hygiene.  

In the case of constrained prostheses it is especially important to have strong initial fixation to the glenoid and this is achieved using screws and cement. For the long term survival of the device, bone growth is required for secondary fixation. It is important that as little bone as possible is removed from the glenoid to maintain the overall strength of the glenoid bone structure.  

The coracoid process is freed up when the rotator cuff muscle group is resected and it is possible to utilize this structure as an extra point of attachment for the glenoid fixation. A plate is fixed to the primary glenoid attachment and a screw fixes the plate to the neck of the coracoid process (Fig 6). The only other design that attempted to have an offset fixation point is that of Kölbel 1987, where he used a flange bolted to the base of the scapula spine 15.

 

Fig 6. Showing the Quad-Point glenoid fixation system, notice the coracoid plate and screw.  

  Quad-Point TSR

The design utilizes some of the concepts used in Depuy’s Delta III reverse total shoulder replacement.  Instead of having a Hydroxy Apatite (HA) coated central stem and four glenoid screws, (where two are locked in place and two have a locating window of 20 degrees) 16, the Quad-Point TSR design uses a cemented central stem and two locating screws, (one superiorly and one inferiorly). A coracoid plate is fixed to the titanium housing using two screws and it is attached to the neck of the coracoid process using one cortical screw (Fig 6).

 

The design strengths (see Table 1):

  • The centre of rotation is located close to the glenoid face.

  • The primary fixation is with three screw points and a cemented central stem

  • The screws that are inserted into the glenoid can be locked at variable angles.

  • The UHMWPE socket prevents the glenoid screws from loosening.

  • The amount of bone removed is low

  • The volume of the prosthesis is low at the proximal end of the humerus component, which reduces the amount of skin needed to close the wound.

 

The design weaknesses:

  • There is little area available for bone in-growth required for secondary fixation.

 

   Hybrid-Screw TSR

The design is a modification of the Bayley-Walker TSR, (Fig 1), with the addition of cement fixation, a coracoid fixation screw, modularity in the humeral component and suture holes. To reduce the possibility of the glenoid splitting, while inserting the central screw, the central stem is threaded only at the end.  This provides the required space for the three-cemented screws peripherally positioned to the central screw. The screws serve two functions one is to facilitate glenoid fixation and the other to lock the coracoid plate into place. Once the system is assembled the glenoid component is fixed to the coracoid process by a cortical screw. Finally the ball is locked into place.

 

The design strengths (see Table 1):

  • A central screw accesses the cortical bone at the vault of the scapula.

  • A large area for bone in-growth is available

 

The design weaknesses:

  • A large offset of the centre of rotation from the glenoid

  • Poor utilization of the bone located around the glenoid cavity because of the fixed angles of the cemented screws.

 

   Central-Peg TSR

This design is a simpler version of the Hybrid-Screw TSR. The idea is to reduce the complexity of the assembly process and the risk of splitting the glenoid. To achieve the above the central screw is replaced with a parallel central peg. 

   Humeral fixation

Regardless of the type of humeral component fixation, aseptic loosening remains remarkably uncommon at this interface. The design given for all three TSRs is a standard tapered stem with three flutes. The stem is fitted with a Morse taper, which locks together with a humeral stem extension. Cortical support plates, are incorporated into the distal end of the humeral stem extension (Fig 7) and (Fig 8). These may be required in cases where an extended proximal humeral resection is necessary or poor bone stock is present. The surfaces of the plates are grooved to accommodate additional surgical tensioning cable. They are also treated to promote osseointergration.

 

 

Fig 7. Humeral fixation  

  Modularity

In cases with a PHS, the level of resection differs for every patient. A surgeon has access to a variety of length humeral stems and shaft extensions in case the resection margin is not well defined and more bone needs to be resected.  

 

Fig 8. The Humeral modular system


  Conclusions:

The number of cases requiring upper limb salvage is low and as a result a constrained total shoulder system has not been made available off the shelf. The Quad-Point TSR design shows the potential to become an inexpensive modular system that will cover the needs of patients with PHS. Of the three-glenoid fixation designs the Quad-Point TSR design stands out as possibly having the highest long term fixation strength of the three. The moment transferred to the glenoid will be lower than that of the other two designs due to an offset of only 18mm of the centre of rotation from the glenoid face.  Also the volume of bone removed is low and the two-glenoid screws are able to access bone superiorly and inferiorly to the glenoid. The volume of the prosthesis is lower than the other two designs, which facilitate ease of wound closure.  

Acknowledgements:

The authors would like to thank the technical staff of the Mechanical Engineering Department at UCT, the orthopaedic surgeons of Vincent Pallotti Hospital and the staff of ISIQU Orthopaedics. Their contributions in their field of expertise brought this project to completion. Also a special thanks to ISIQU Orthopaedics for the financial support.

Reference :

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  9. McElwain, JP and English, E. The early results of porous-coated total shouder   arthroplasty. Clinical Orthopaedics, 216:217, 1987.
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  12. Author. 2005.
  13. Couteau, B, Mansat, P, Estivalezes, E, Darmana, R, Mansat, M and Egan, J. Finite    element analysis of the mechanical behavior of a scapula implanted with a    glenoid prosthesis. Clinical Biomechanics, 16:566–575, 2001.
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  15. Kölbel, R, Helbig, B and Blauth, W. Shoulder Replacement. Springerverlag:   New York, 1987.
  16. Boileau, P, Watkinson, DJ, Hatzidakis, AM and Balg, F.   Grammont reverse prosthesis: Design, rationale, and biomechanics. Journal of   Shoulder and Elbow Surgery, 14(1S):147S–161S, 2005.

 

This is a peer reviewed paper 

Please cite as : Vicatos George : A Comparison Study Between Three Constrained Total Shoulder Replacements Designed for Proximal Humeral Sarcoma Limb Salvage Cases.

J.Orthopaedics 2007;4(3)e22

URL: http://www.jortho.org/2007/4/3/e22

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