various prosthesis designs and their indications

 

A 72-year-old female presents with a 2-year history of progressive knee pain. She has tried braces and anti-inflammatories with limited benefit. Her primary care physician prescribed a course of physical therapy and corticosteroid injections which did not provide pain relief. The patient is now dependent on a cane, has been gaining weight due to difficulty ambulating, and has trouble sleeping. X-rays of the knee show moderate arthritis, and physical examination reveals joint-line tenderness and knee pain that is exacerbated by range of motion of the hip.

The next step in management would involve:

1. Another series of corticosteroid injections

2. Since steroids failed, a series of hyaluronan injections

3. Additional x-rays of the hip

4. Arthroscopy to address the likely meniscus tear

5. Total knee replacement

 

Discussion

The correct answer is (C). Since the patient complains of knee pain and intra-articular injections did not provide relief, it is possible that her severe pain is referred pain such as from an ipsilateral arthritic hip or radicular pain originating from the spine. Examination of the hip and spine should be performed, and additional radiographs of the hip should be considered in order to rule out referred pain.

Upon further questioning it is revealed that the patient had trauma 30 years ago following a motor vehicle crash whereby she sustained a comminuted patellar fracture and underwent patellectomy. If conservative measures fail to provide pain relief, the recommended surgical treatment of her knee pain is:

1. Unicompartmental knee arthroplasty

2. Posterior cruciate retaining knee arthroplasty

3. Posterior stabilized knee arthroplasty

4. Constrained knee arthroplasty

5. Hinged design knee arthroplasty

 

Discussion

The correct answer is (C). In the absence of a fully functioning extensor mechanism, including patients with previous patellectomies, the use of a posterior stabilized prosthesis is indicated to prevent anterior tibial instability that would typically be constrained by the presence of the patella and a tighter extensor mechanism. A cruciate retaining device can be performed, however, studies suggest a higher failure rate in those patients.

The patient inquires regarding the possibility of press-fit components for her knee replacement as she has heard that bone cement is toxic. Cementless knee replacements have:

1. Increased infection risk

2. Increased polyethylene wear

3. Increased risk of fracture and notching

4. Increased risk for revision

5. Increased satisfaction and outcomes

 

Discussion

The correct answer is (D). Cementless knee replacements are associated with increased tibial loosening and associated need for revision. There is no established difference in infection, polyethylene wear, or patient satisfaction.

The patient has seen an advertisement about total knee replacements and has questions about an all-polyethylene tibial component. You tell her that these components result in:

1. Easier customization during the case

2. Increased rates of osteolysis

3. Decreased backside wear

4. Increased range of motion

5. Increased rate of revision

 

Discussion

The correct answer is (C). All-polyethylene tibial components appear to be superior to metal backed components in terms of wear and revision rates. Modular components are associated with increased backside wear due to micromotion between the insert and the metal base plate. Nevertheless, metal backed components allow for modularity and thus increased customization during the case, while monoblock tibial components require determination of size before cementing.

The patient undergoes primary posterior stabilized TKR. Intraoperatively after performing the tibial and femoral cuts including chamfers and PS box cut, while balancing the knee you notice that the knee is balanced in flexion, but she continues to have a flexion contracture. What is the next most logical step to correct this imbalance?

1. Resect more proximal tibia

2. Resect more distal femur

3. Downsize the femoral component to gain more range of motion

4. Remove posterior osteophytes and release posterior capsule

Discussion

The correct answer is (D). When balancing a primary total knee replacement with a gap technique, it is important to create a rectangular space that is symmetric from medial to lateral and in flexion and extension. If there is sufficient range of motion in flexion, but there remains a “flexion contracture”, this means that the knee cannot fully extend and the “extension” gap is tight while the flexion gap is appropriate. This means that the space between the femur is smaller when the knee is in 0 degrees of flexion in comparison to when the knee is at 90 degrees of flexion (see Fig. 7–7). In order to increase the extension gap, one must either remove posterior osteophytes and release posterior capsule (the osteophytes tent the capsule and create a tighter extension gap), recess the PCL in a PCL retaining knee, or remove more distal femoral bone. In this patient who has a PCL sacrificing knee, it is not an option to recess the PCL. Resecting more distal femoral bone is an option to correct the imbalance but requires applying the cutting jig again and recutting the chamfers if these have already been cut. The most simple next step at this stage of the total knee replacement would be to carefully remove the posterior femoral osteophytes and release the posterior capsule from the distal femur using electrocautery.

 

 

 

Figure 7–7

 

The patient subsequently healed uneventfully following posterior stabilized primary total knee replacement. She returns to clinic 8 months following her surgery with anterior knee pain associated with a clunking sensation when she gets up from a seated position, ascends stairs, or straightens her knee. What is likely the source of this problem?

1. Component malrotation

2. Excessively thick polyethylene insert

3. Early polyethylene wear

4. Fracture of the post on the polyethylene insert

5. Patella clunk syndrome

 

Discussion

The correct answer is (E). Due to the design of the posterior stabilized knee replacement with a large box cut in the femoral component, patella clunk syndrome can sometimes be observed wherein a nodule along the quadriceps surface catches over the trochlear notch when the knee is moving from an extended position to 30–45 degrees of flexion. Symptoms can be reproduced with resisted extension.

The recommended treatment for this patient is:

1. Observation

2. Manipulation under anesthesia

3. Arthroscopic synovectomy

4. Revision to a CR knee

5. Revision to stemmed constrained components

 

Discussion

The correct answer is (C). As discussed above, patella clunk is thought to be due to soft tissue constrained onto the femoral component. Arthroscopic synovectomy, excision of the scar tissue, and synovial fold above the patella result in improvement of symptoms. Further revision is not indicated at this time in the setting of well-aligned components.

When evaluating a painful total knee replacement, if there is concern for malrotation of the components, the radiographic modality of choice is:

1. Standing 3-ft x-rays

2. Flexion x-ray of the knee

3. Skyline view of the knee

4. CT imaging

5. MRI imaging

 

Discussion

The correct answer is (D). Although, axial radiographs may be used to detect axial

rotation of the femoral component, CT is most commonly used for the purpose of measuring component rotation. MRI is also acceptable but would not typically be the first line in an otherwise normal patient.

 

Helpful Tip:

Cruciate retaining and posterior stabilized knee designs are competing trends with no significant difference in long-term outcome, range of motion, and stability in the hands of a good technician. Nevertheless, comfort with the design and understanding of degree of constraint and associated modularity is imperative in a successful total knee arthroplasty.

 

Objectives: Did you learn...?

 

 

Recognize various prosthesis designs and their indications? Understand the spectrum of constraint in TKR?

 

Understand how to balance a primary TKR?