Press Fit vs. Cemented Knee Implants
Orthopedic surgeons select knee implants that either require acrylic bone cement (polymethylmethacrylate or PMMA) to secure the implant or press-fit knee implants in which the patients' bone grows into porous coated baseplates. Implants that rely on bone integration are now favored in hip, shoulder, spine and dental implants because cemented fixations are often weaker and subject to loosening. For active and athletic patients, cemented implants are generally not satisfactory because the cement interface may not withstand the shear (side to side) stresses that occur during activities such as golf, racquetball and skiing. Bone density typically receeds beneath PMMA over time. Table 1 lists the advantages and disadvantages of implants that rely on either bone ingrowth or bone cement.
Table 1. Comparison between implants that rely on either bone ingrowth or bone cement for fixation.
| Advantages | Disadvantages | |
| Bone Ingrowth | Stronger bond long term Infection rate very, very low Operation time reduced Late revision typically exchanges liner only |
More technically demanding for surgical team |
| Bone Cement | Surgical technique is more forgiving Good track record for non-athletic recipients |
High temperatures of bone cement may damage surrounding bone cells May provoke an immune response May loosen over time Small particles of bone cement embed in poly, accelerating wear Late revisions often require removal of all components and cement |
Our press-fit surgical technique is meticulous in preparing the bone to ensure bio-integration. This results in a glove-like fit between the bone and implant, encouraging bone ingrowth. Ingrowth results in a stronger interface between the bone and implant that is biologically responsive, not susceptible to loosening from vigorous activities. Figures 3a and 3b show the precise cutting required for press-fit implants.
Bone Coverage
Full bone coverage or capping by an implant improves bone integration and reseals the bone environment. As a rule, male bone structure requires broader coverage patterns than female. As a result, male preference implants are broader than female. A range of 1 mm overcapping to 2 mm undercapping is optimal (Figure 4).
Minimally Invasive Surgery
Marketing executives have falsely equated minimally invasive surgery (MIS) to techniques with a smaller incision. Consumers should be cautious when reading potentially skewed marketing information about MIS. Figure 8 shows a photograph of the incision used for a full knee technique. Surgeons routinely strive for the “minimally invasive objectives of:
- Gentle tissue handling (skin, muscle, tendon, etc)
- Maximum bone preservation
- Preservation of ligament function
- Optimal alignment for hip, knee, and ankle co-functions
- Economical incisions (Figure 8)
- Minimized tourniquet times
- Early, full weight bearing (e.g. 4 hours after surgery)
- Control of bleeding and rare use of blood transfusion
- Very low risk of repeat surgery
MIS emphasis has been on “half-knee” or "uni-compartmental" replacement utilizing acrylic cement. These partial knee replacements often do not achieve our surgical objectives such as optimal alignment and healthy bone integration. Some surgeons have correctly characterized partial implants as the “first stage” of a full implant. Most observers agree that a cemented half-knee replacement is not suitable for the greater demands of athletic patients.
PCL sparing vs. PCL sacrificing
There are two basic implant types available for total knee surgeons. These are posterior cruciate ligament retaining (CR) and posterior cruciate ligament sacrificing (PS). Dr. Rosenberg has been implanting press-fit, CR implants for over 25 years in athletic patients. Figure 11 shows the preservation of the PCL during surgery.
The posterior cruciate ligament (PCL) stabilizes and contributes balance function (proprioception) to the knee joint and should be retained whenever possible. By protecting the PCL and balancing the soft tissues surrounding the knee, patients are able to safely engage in a variety of sports like golfing, skiing and hiking. Table 1 lists the key features of CR and PS implants.
Table 2. Important features of PCL retaining (CR) and PCL sacrificing (PS) implants.
| Description | PCL Retaining (CR) | PCL Sacrificing (PS) |
|
Sacrifices PCL |
No |
Yes |
|
Ligament balance in extension |
Yes |
Yes |
|
Ligament balance in flexion |
Yes |
No |
|
Maximum bone preservation |
Yes |
No |
|
Optimizes implant wear |
Yes |
No |
|
Greater loosening risk in athletic patients |
No |
Yes |
Surgical Concepts
We have achieved 99% success in bone integration by carefully implementing the following principles:
- Neutral alignment (see Figures 12 & 13)
- Ligaments stable in extension and in flexion
- Ligaments stable medial to lateral (side to side)
- PCL (posterior cruciate ligament) preserved
- Immediate weight bearing
- Maximized bone preservation
Bone integration also requires a good biological response from the patient's own bone. Bone nutrition including supplementation and anti-inflammatory nutrition encourage the recipient's bone response. Bone density drugs such as Fosamax and Boniva are not recommended and have even led to pathologic fractures. NSAID's like ibuprofen and Celebrex are discouraged because they may inhibit bone formation. Smoking is perhaps the greatest inhibitor of bone formation and therefore most regular smokers are not candidates for bone integration implants.
Bone Health
Healthy bone is your "next line of defense". Arthritic patients with good bone quality experience less pain, some report no pain at all.
