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They still represent the most common type of implant (fig. 9).
Figs. 9-10: Total anatomical shoulder prosthesis: humeral component with stem, glenoid component with metal back; Resurfacing prosthesis
In recent decades, more conservative and less invasive humeral components have been developed. This means removing less bone and filling a shorter length of the humerus. These prostheses that are more respectful of the bone structure are the “resurfacing prostheses” and “hemicephalic prostheses” (or “half head”). They are indicated when the damage to the joint surface is not too extensive, the proximal humerus is not deformed and the bone consistency is satisfactory. Therefore they are implanted in a younger age group than the average prosthesis.
A resurfacing prosthesis is a kind of metal cap that goes to cover the humeral head after having prepared it with spherical burs.
A hemiphalic prosthesis is instead put on after having resected a thin layer of the humeral head and is equipped with a short taproot that does not get to penetrate the medullary canal.
These prostheses have been designed with the aim of sacrificing less bone, thus preserving a more abundant bone “stock” for possible future re-operations, but they can only be implanted in a minority of cases. Even the materials of the prostheses themselves have undergone a strong innovation in recent years, with the development of more biocompatible and respectful coatings of the host tissue, both bone and cartilage: the stems are more integrated by the host bone tissue, which literally “clings ”In a natural way to the prosthetic component, stabilizing it, allowing us not to use cement for fixation, and the sliding surfaces are more similar to articular cartilage and therefore delicate on the latter, such as pyrocarbon.
Glenoid component – When the articular surface of the scapula (the glenoid or glenoid) has also degenerated, a scapular component with a concave surface is coupled to the three types of humeral prostheses already seen – anatomical, covering, hemicephalic metal base (the “metal back”) on which a sort of polyethylene shield fits (fig. 9 and 13); or it can be entirely in polyethylene (fig. 12).
Figs. 12-13: Hemiphalic prosthesis with glenoid component in polyethylene (radiolucent) and with glenoid component in metal (metal back) and polyethylene
Reverse prosthesis – Another model of total prosthesis that has been gaining ground in recent years and which deserves a separate discussion is the so-called “reverse prosthesis” (fig. 14).
Fig. 14 – Osteoarthritis of the shoulder treated with the short-stem, non-cemented inverse prosthesis implant, saving host bone tissue
This implant bears this name because the geometries are inverted with respect to the natural joint: a convex, hemispherical component is attached to the scapula, while the concave surface is located on the humerus side. For complex biomechanical reasons this system is indicated in cases where the function of the rotator cuff is compromised due to irreparable massive injury. In these situations the other types of prostheses would not allow to recover a satisfactory active movement. An indispensable condition for the functionality of the inverse prosthesis is that the deltoid is efficient.
The inverse prosthesis is generally indicated in patients who are no longer young, whose rotator cuff has a large lesion, or who is perhaps so degenerated as to suggest that in a few years they will suffer a massive lesion.
Also for shoulder prostheses we use the most modern technologies of calculation and pre-operative computer simulation: before the surgery the prosthetic components or bone grafts are virtually positioned on the processing of the patient’s diagnostic images, in order to evaluate with accuracy of any critical issues encountered during surgery. This allows to obtain more and more precision, longer duration and better results of the prosthetic implants used.
When you are hospitalized, the main tests necessary to evaluate the shoulder (a CT scan to study the bone structures, often also an MRI to judge the condition of the rotator cuff) have usually already been performed and evaluated in the outpatient visits.
Blood tests and a visit to the anesthetist are performed, who explains to the patient the methods of anesthesia (locoregional, general or mixed). If there is no need for further investigations, the intervention planning is confirmed.
The duration of the operation varies, however in most cases it ranges from 90 ‘to 120’. The stay in the operating room also includes a preliminary preparation time, usually of at least one hour, and a surveillance period after the end of the operation.
Only after the surgery is it established how the shoulder should be mobilized and rehabilitated. In most cases, a light brace with arm sling is sufficient for shoulder protection.
Except in particular situations, the most basic movements of the limb and also of the shoulder can be started in the first few days (bring your hand to your face, use cutlery, etc.).
Discharge occurs when the general conditions and blood tests are stabilized: it generally takes 4-5 days after the surgery. Upon discharge, an appointment is set for the first check-up in the clinic.
As in all the activities we carry out on a daily basis (working, playing sports, traveling …), adverse events can also occur during or after surgery, which are called complications.
After a shoulder replacement surgery they can be of various types, fortunately they are rare events.
They include the formation of hematomas (i.e. blood collections in the operated shoulder); paralysis of a nerve (when they occur they are almost always consequent to stretching and therefore transient), fracture of the humerus or scapula (failure of the osteoporotic bone during the preparation or insertion of a component of the prosthesis).
As with all interventions, especially when they include the application of artificial implants, there is a risk of infection despite antibiotic prophylaxis.
At a long distance, as with all prostheses, the problem of mobilization may occur, ie the detachment of a prosthetic component from the bone that hosts it. This occurrence is rare for the humeral components, more likely for the glenoid components.
The objectives that arise when deciding to put on a shoulder prosthesis are the elimination of pain (which is obtained in more than 90% of cases) and an improvement in movements, the extent of which is however variable: for example, a gesture such as bringing the hand to touch the back from below is recoverable only in a minority of cases. The starting condition strongly influences the functional result that can be achieved. In general, it can be said that the most brilliant results are obtained in the circumscribed necrosis of the humeral head, while the least satisfactory results are obtained in the outcome of fractures.
With a shoulder prosthesis you should not practice (for life) heavy activities, such as moving heavy weights, using tools (for example hoes, spades, etc.), playing sports that require sudden shoulder strain (tennis, golf …). In addition, the risk of trauma and falls must be carefully avoided, because a possible fracture of the bone in correspondence with the prosthesis can represent a very complex problem.
Authors: Dr. Roberto Rotini and Dr. Graziano Bettelli, Complex Structure of Shoulder and Elbow Surgery, Rizzoli Orthopedic Institute.