When a joint, such as the hip or shoulder, becomes impaired due to arthritis, disease or trauma, it is sometimes necessary to replace all or part of the joint with a prosthesis to restore function. For instance, hip replacement, where a prosthesis is provided to replace the femoral head and in some cases all or part of the acetabulum, has become a common procedure to treat femoral head fractures and arthritis in elderly patients. As a result of anatomical constraints and challenges in the shoulder, shoulder implants have historically been much less successful and less common than hip replacements. Recently, however, shoulder arthroplasty has emerged as an accepted treatment for severe arthritis and humeral head fractures.
As a consequence of the increasing acceptance of shoulder prostheses, many different devices have been developed to address various problems that have arisen and to offer additional benefits and features. In the simplest form, a shoulder prosthesis is formed as a single piece with a head to articulate with the glenoid cavity, and a stem to extend down the medullary canal of the humerous and support the head. While simple to construct, unitary implants do not offer any adjustability to accommodate the natural variations in size and geometry that occur among joints of different patients. To accommodate these variations, a large stock of devices must be manufactured and maintained to insure that an adequate match can be achieved during an operation. Stocking the large number of devices is a significant expense with one-piece designs, and in some cases a surgeon may not be provided with sufficient flexibility to achieve an ideal fit to the patient.
To avoid the expense of maintaining a large stock of single-piece prosthetics and to provide increased flexibility to surgeons, many shoulder implant makers have gone to a modular design that is assembled during the operation from two or three pieces. These pieces include a head to articulate with the glenoid and a stem structure on which the head is mounted and secured to the bone. In some cases, the stem includes a separate body portion disposed between the head and an intermedullary portion of the stem that extends down the medullary canal. By utilizing a modular design, a wide variety of devices can be assembled from only a few pieces, thus providing increased flexibility to accommodate anatomical variation and eliminating much of the cost associated with maintaining a large selection of one-piece devices.
One drawback of existing modular implants is the difficulty of reliably and easily attaching the pieces together. With existing designs, the pieces are most commonly held together with a taper-lock structure. In particular, the backside of the head is provided with a male or female taper, and a mating structure is provided on the top of the stem. After selecting the appropriate components, the surgeon places the head on the stem and drives the pieces together to lock them in place. Unfortunately, because the components are held together only by friction, it is possible for them to become loosened or dislocated after installation, in which case another operation must be performed to restore the implant.
Another drawback with taper-lock modular designs is that it can be difficult to disassemble an implant and install a new head without removing the stem from the bone. In particular, it is often difficult to impart enough force to the head to separate the taper-lock without dislodging the stem from the bone at the same time. Moreover, to be removed, the head must be lifted away from the stem by the length of the taper and, thus, the joint must be dislocated to permit the necessary separation. As a result, the recovery time is greatly extended over what would be required if dislocation were not necessary.
Another common feature of many existing taper-lock designs is a proximal flange attached to the top of the stem adjacent the head. This flange prevents the implant from subsiding down into the femur and avoids the resulting upward force on the bottom of the head which would tend to separate the taper lock. Unfortunately, over time, bony ingrowth can occur around the underside of the flange and the sides of the stem. Although this bony ingrowth is beneficial in that it helps to stabilize the implant, it also makes the implant much more difficult to remove when a revision is necessary. In particular, the flange blocks the surgeon from slipping a chisel down the bone adjacent the sides of the implant to separate the implant from the bone. As a result, a significant amount of bone may be dislodged with the implant, making it more difficult to secure the replacement implant.
The flanges used on many implants are also problematic because they decrease the thickness available for the head. The natural humeral head is typically 16-18 mm thick. Typical flanges are around 3 mm thick and, with a taper-lock device, a gap of approximately 2 mm must be left between the bottom of the head and the collar to accommodate the machining tolerances in the taper. As a result, the implant head is often significantly thinner than the original anatomy. This can limit joint mobility and increase the chance of dislocation.
In addition to the specific drawbacks associated with various existing implant designs, there are a number of general problems inherent in shoulder replacements. In particular, it is generally difficult to establish the proper position and orientation for the implant in the humerus. One of the more important variables is the rotational position, or retroversion, of the head on the humerus. Anatomically, the average retroversion between a plane defined by the perimeter of the anatomical head and the axis of the flexed forearm is approximately 30-degrees. Unfortunately, with existing implants and techniques for their installation, it has been very difficult to reliably reproduce desired retroversion. Establishing correct retroversion is important because incorrect retroversion can lead to problems with subsequent dislocation.
In addition to the retroversion of the implant, it is necessary to establish the correct height of the implant on the humeral shaft. With existing designs, the surgeon slips the stem into the medullary canal and makes an educated guess at the proper height. Excess height may create too much tension in the deltoid, while inserting the implant too far down the humerus can result in deltoid lag. Similarly, the offset of the face of the head relative to the stem must be established correctly or excess or insufficient tension in the rotator cuff may be created. Unfortunately, with existing designs there is no way to evaluate implant height or head offset prior to final installation, after which correction is difficult.