Knee arthroplasty: What radiologists should know.

Transcription

1 Knee arthroplasty: What radiologists should know. Poster No.: C-2174 Congress: ECR 2014 Type: Educational Exhibit Authors: J. Araújo 1, J. Pires 2, J. Oliveira 3, C. Macedo 1, S. Magalhães 1, I. Ferreira 1, R. Maia 1, M. Ribeiro 1 ; 1 Porto/PT, 2 Coimbra/PT, 3 Portugal/PT Keywords: DOI: Musculoskeletal bone, Musculoskeletal system, Trauma, CT, Conventional radiography, Education, Prostheses /ecr2014/C-2174 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 16

2 Learning objectives To describe the radiologic aspects of the different prosthesis, normal postoperative appearance and most common complications. Background Osteoarthritis Fig. 1 on page 4 is the main cause for knee arthroplasty, followed by rheumatoid or other inflammatory arthritis and osteonecrosis. The main objectives of arthoplasty are the relief of pain and improvement of function. Metal, ceramics, polyethylene and bone cement are the most frequently used materials for knee arthroplasty. Metal: the properties of metals, such as strength, durability and biocompatibility, depend on their compositions and structure. The two main metals utilized in the knee prosthesis are cobalt chromium and titanium. Ceramics: It has better strength and resistance to corrosion and fatigue, when compared to metal. However, it is difficult to use in the knee joint because of its complex biomechanics and structural congruency. Besides, it has to be fixed without bone cement. Polyethylene: Is used as a spacer between tibia and femur and as a patellar component. Its advantages are the high resistance, elasticity and flexibility. The most common complication is the wear. Bone cement: Is the most frequent material used to fix the prosthesis to bone. The design of the knee prosthesis is a very important aspect, as it not only dictates the durability of the knee prosthesis, but also the quality of the patiet s life. The above listed aspects are the main objectives of the prosthesis design: Preserve the bone as much as possible Minimize the chances of loosening Minimize the production of wear debris Page 2 of 16

3 The design of the knee prosthesis is based on its anatomy and function. The factors to be considered in implant design are the following: Congruence between components (polyethylene has an important role) Motion constraint between the components, where the unconstrained motion implant have more risk of loosening but less of wear: 1. Unconstrained (its stability depends a lot on the surrounding ligaments) 2. Constrained (any implant that restricts severely valgus-varus could be classified as a constrained type) 3. Semiconstrained (most common): the key point is the maintenence of the posterior cruciate ligament (PCL), as it plays an important rule in the roolback, posterior stability and proprioception. It reduces the wear, the loosening and the stress. Posterior cruciate: retained type (CR type) Posterior cruciate: substituting type (PS type) There are two types of fixation methods: Cement type: the most popular method of fixation. It has good results but the risk ok loosening is bigger in active patients. Cementless type: the prosthesis have porous coating. In this type, the fixation is permanent, more bones are preserved and no problems are caused due to the bone cement. However, its success is determined by the reliability of the bone ingrowth, as it needs a lot of stability in the first postoperative days. It is associated with more fixation problems in the first month and a highter probability of complications too. It is contraindicated in osteophenia/osteoporosis. When it comes to the younger and more active patients, cementless type is the one that is recommended. In Fig. 2 on page 5 we can see each component of the prothesis : Femur: the design depends on the overall shape, the method of fixation, CR or PS type, and the motion constraint. Tibia: the design depends on the overall shape, too, the shape of the stem and to the fixation method of the polyethylene to the tibial plate. Spacer (polyethylene): if the shape is to flat, it will have low conformity and increased wear. Beside, if conformity is too high loosening is increased. Impingment can occur between the patella and the polyethylene due to the patella's baja. If the front of the spacer is diagonaly shaped, the risk is Page 3 of 16

4 decreased. The spacer shouldn't be thinner than 9mm as it would increase the wear. Patella: a successful patellofemoral articulation must be designed to be functional and endure high stress. It could have different shapes and currently the most used material is polyethylene. There are two types of knee prosthesis: total knee artroplasty Fig. 2 on page 5: used in the most severely degenerated and deformed knee unicondilar prosthesis Fig. 3 on page 6: frequently used in singlecompartment arthritis, more commonly in the medial compartment. Images for this section: Page 4 of 16

5 Fig. 1: Bilateral osteoarthrosis, advanced on medial femoral-tibial compartment. Condrocalcinosis signs Page 5 of 16

6 Fig. 2: A- Femoral component B- Patella (polyethylene) C- Spacer(polyethylene) D- Tibial component Fig. 3: On the left, osteoarthrosis more exuberant in the medial component of the right knee. On the right an unicondilar prosthesis. Page 6 of 16

7 Findings and procedure details The normal radiologic aspects most commonly seen on x-ray follow up of the arthroplasty are the following: The development of a thin (less then 2mm) lucency, between the bone and the cement, and may also develop a sclerotic line around that lucency. These aspects are more frequent on the tibial component. The stress shielding that occurs due to non-uniform transfer of load, usually in the distal femur, leads to a sclerotic band. This may be seen on the lateral x-ray in the posterior aspect of the femur, often associated with increased bone resorption in the non-weight bearing portion. A crucial point in the analysis of the prosthesis is the presence of lucent lines, which may be due to different complications. It is extremely important to correlate these lucent lines with previous imagiologic exams. Prosthetic and periprosthetic complications are the following: Instability with dislocation Fig. 4 on page 10 is the second most common cause to revise surgery, preceded only by infection. Dislocation is when we have opening more than 2mm medial and 3mm lateral in extension, and a 3mm medial and 4mm lateral in flexion. AP instability is defined when tibia can be displaced more than 5mm or when the joint can be dislocated. Instability increase wear and loosening. Infection Fig. 5 on page 14 : can be the most awful complication and it usually requires revision. Its incidence is around 0.5-2%. X-ray may not show abnormal findings at first, but bone destruction with irregular borders, loosening of the prosthesis and new periosteal bone formation can be found and, in some cases, also gas in the soft tissues. The risk of loosening increases and generally the loosening affects all the components, which is different from aseptic loosening that normally affects a single component. Aseptic loosening Fig. 6 on page 9 : is the separation of the artificial prosthesis from the bone, causing instability. The incidence of loosening increases when the bone is weak or osteopenic. The incidence is high in high-flex designs, constrained prosthesis and cementless type of fixation. Another cause for aseptic loosening is the misaligment of the prosthesis. In the X-ray, a progressive increase in periprosthetic lucency or lucent zones, greater than 2mm, are evidence of loosening. Cement fragmentations, frature migration or change in position of the prosthesis are also diagnostic of loosening. A gap of 2mm between the cement and bone is considered Page 7 of 16

8 acceptable. Loosening is more common around tibial component and fluoroscopy is sometimes preferred to make the correct diagnosis. Wear is responsible for 25% of all revisions. When wear occurs, the vicious cycle of osteolysis starts leading to loosening. Wear can occur uniformly but tends to be eccentric on the medial or posteromedial side. Initial wear is confined to the PE and tends to progress. Low conformity or flat prosthesic design, titatanium material and PE thinner then 6mm increases the risk of wear. A stress X-ray is useful for the diagnosis, but only when the wear progresses to the metal. Ultrasonography has been shown to be helpful in evaluating the PE thickness. Osteolysis Fig. 7 on page 11 is the most common complication of arthroplasty. It's the formation of wear particles debris that occurs due to chronic foreign body inflammatory reaction to particles and cement. It is also known as an agressive granulomatosis. Metal materials also cause osteolysis, particularly the titanium, and are more common in the tibial component. Osteolysis also induces a synovitis Fig. 8 on page 12. On X-ray, osteolysis appears as a focal or linear periprosthetic lucencies. Progressive osteolysis leads to a prosthetic loosening. Periprosthetic fracture Fig. 9 on page 13 : osteopenia and intra-operative factors are the main causes of periprosthetic fracture. It s more common at the medial femoral condyle. Images for this section: Page 8 of 16

9 Fig. 2: A- Femoral component B- Patella (polyethylene) C- Spacer(polyethylene) D- Tibial component Fig. 6: Asseptic loosening, with difuse lucencies Page 9 of 16

10 Fig. 3: On the left, osteoarthrosis more exuberant in the medial component of the right knee. On the right an unicondilar prosthesis. Page 10 of 16

11 Fig. 4: The right tibial component is not correctly alignment with linear lucence between the medial condyle and the tibial prosthesis compartment Page 11 of 16

12 Fig. 7: The exuberant osteolysis around the tibial component with fracture of femoral condyles. Page 12 of 16

13 Fig. 8: Difuse hypoechogenic sinovial thickening Page 13 of 16

14 Fig. 9: Periprosthesis fracture Page 14 of 16

15 Fig. 5: Radiolucence line between the bone and the femoral and tibial prosthetic component. Page 15 of 16

16 Conclusion The number of patients requiring knee arthroplasty is increasing. Radiologists must be aware of its normal appearance and the possible complications associated in order to diagnose them. It is also important to have a methodic analysis and description of the prosthesis components and a comparization to initial post-op radiographs. Personal information References Taljanovic MS et al. Joint Arthroplasties and Prostheses. Knee Joint Arthroplasty-Springer Berlin Heidelberg (2014) Postoperative Radiologic Findings. BJ Manaster. AJR 1995; 165: Bone and Joint Imaging, 3e [Hardcover]: Donald Resnik Page 16 of 16