Uncemented Vs Cemented Hip Replacement
Hip replacement surgeries involve placing a metallic stem (femoral component) inside the canal of the upper part of the thigh bone. A metallic head is placed on top of the femoral stem that duplicates the function of the natural head of the femur. Similarly, a metallic cup is placed in the natural socket of the hip joint formed by the acetabulum.
Cemented and uncemented hip arthroplasties differ in how the prosthetic implant is fixed to the natural bone. A cemented hip arthroplasty relies on bone cement to fix the implants to the bone interface. Uncemented hip replacements rely on bone on-growth over specially designed implants.
Cemented Hip Replacement
A cemented hip replacement uses a polymethylmethacrylate (PMMA) bone cement to fix the implant surface to the bone. The bone cement creates a grout and fixes the implant by acting as an interlocking surface between the implant and the bone rather than simply gluing the implant to the bone.
The bone cement is made by mixing powder and liquid components. The mixing and cementing techniques have evolved in the past decades to achieve maximum fixation. The modern mixing in the operating room involves the use of a vacuum mixer. The bone cement hardens in about 10 minutes in the OR temperature.
The modern technique of cementing involves:
- Preparation of the femoral canal using serial broaching and using a pulsatile lavage to wash the canal and remove debris.
- A cement restrictor is placed at an appropriate distance to prevent the cement from traveling down the canal.
- The cement is filled in the canal using a cement gun that pressurizes the cement injection in the canal. The gun ensures the cement is not eviscerated by the bleeding in the canal.
- The femoral stem is inserted to pressurize the cement and the tapered stem design ensures the cement pressure is maintained during the insertion. A proximal seal is made to ensure adequate pressure during the polymerization of the cement.
- The acetabulum cementing is rarely done but is similarly done to ensure adequate fixation. Beads may be used at the cement implant prosthesis to ensure an even cement distribution and to prevent bottoming out of the cement.
The cemented hip replacements are indicated only in limited cases:
- Osteopenic or osteoporotic patients have thin and porous bones. The porous bones may not support the uncemented implant due to poor bone ingrowth. A cemented implant has a better fixation as a result of cement penetration of the porous bone.
- Patients with a history of bone irradiation as a result of any malignancy who may need a hip replacement are best managed with a cemented hip replacement. The irradiated bone has poor potential for bone on-growth or bone in-growth.
- Patients with abnormally wide femoral canal are ill-suited for an uncemented hip replacement. The cement mantle ensures adequate fixation in patients with a wide femur (Dorr C).
Uncemented Hip Replacement
Uncemented hip replacement uses specially designed prosthetic components for the bone to implant fixation. The femoral component is usually covered with a porous surface on the upper end. The porous sprayed surface has microscopic contoured troughs and ridges.
The natural bone grows on and in the porous surface creating a gridlock that binds the implant to the bone. Uncemented fixation is the most common method of total hip replacement in the United states.
The operating surgeon prepares the bony femoral canal by broaching and inserting the implant slightly bigger than the final broach. The insertion of a slightly larger implant ensures a press-fit prosthesis that is snuggly fit.
The acetabular component is mostly uncemented unless the patient has poor bone stock or has a history of irradiation. The fixation is usually aided with the screws that help secure the acetabulum shell in place until the bone ingrowth occurs. The uncemented type of fixation is the preferred type of fixation in elderly patients with a good bone stock, young patients, and in the setting of a revision hip replacement surgery.
Comparison of cemented and uncemented fixation
The uncemented prosthetic designs were advanced to overcome the complication faced with cemented fixations. Cemented fixation was liable to cement breakage and subsequent inflammation around the prosthesis. The breakage, micromotion, and inflammation may lead to implant loosening.
Bone cement implantation syndrome (BCIS) is a rare complication associated with cementing during joint replacement surgery. The cement material is hypothesized to travel in the bloodstream which may cause difficulty breathing, and cardiac arrest. Pulsatile lavage is aimed to prevent cement and fat globules from leaking into the bloodstream.
The uncemented fixation is not associated with BCIS and the cement osteolysis is not associated with uncemented methods. A well-positioned uncemented implant fixation gets stronger at the interface with time as bone ingrowth occurs, however, loosening may also be associated with uncemented stems.
With a setting time of 10 mins, the cemented implants are fixed instantaneously and patients are able to bear weight right after the surgery. However, in the case of uncemented stems, only partial weight-bearing is allowed initially as the bone ingrowth takes at least 3 months. However, with recent changes in implant designs and techniques, some surgeons may allow weight-bearing right after surgery even in uncemented fixations.
Positioning of the femoral stem is easier in cemented fixation as the stem position may be individually adjusted during cementing and stem insertion. However, in the case of uncemented fixation, the position is largely determined by the reaming and broaching of the femoral marrow.
There are numerous studies that compare the complications such as stress shielding, peri-prosthetic fractures, long-term survivorship, ease of revision, etc. between uncemented and cemented techniques with variable results.
The type of fixation used in hip replacement is largely determined by the patient’s individual hip anatomy, age, and any history of medical conditions. While cemented fixation is mainly done at the extremes of age, even these patients may be candidates for uncemented fixation depending upon their bone stock.