November 22, 2020
diagnosing osteoarthritis of the knee with a knee radiograph

Diagnosing Osteoarthritis with a Knee Radiograph

One of the most common clinical entities seen by a sports medicine physician is knee pain. In our aging population, the incidence of osteoarthritis is increasing. Many of the cases of knee pain seen in the office are due to osteoarthritis. Sports physicians are typically charged with reading x-rays in office and must be aware of the signs of osteoarthritis of the knee on a radiograph.

Case Question

A 65 year old patient comes in with right knee pain for three months. Pain is localized to the medial compartment of the knee. On examination there is knee crepitus and medial joint line tenderness. Which of the following radiologic findings would you expect to find in a knee with primary osteoarthritis?

A) Subchondral sclerosis

B) Pellegrini Stieda Sign

C) Symmetric joint space narrowing

D) Erosions

Radiographic Findings

Radiographs of the knee should be weight bearing in order to assess for osteoarthritis (2). Three views of the knee are typically ordered. These are bilateral AP, lateral, and sunrise view. This allows the provider to evaluate the medial tibiofemoral compartment, lateral tibiofemoral compartment, and patellofemoral joint (6). When assessing a knee radiograph for osteoarthritis, physicians should look for sclerosis, osteophytes, and joint space narrowing (1).
Joint space narrowing is also seen with osteoarthritis, but is not specific for degenerative joint disease (1). Joint space narrowing rarely occurs homogenously across a joint. Cartilage loss is typically found asymmetrically throughout the joint (2). The most common location for joint space loss in the knee is the medial compartment (2). We use joint space narrowing as an indirect monitor for cartilage loss in the knee (3). However, an MRI can better assess the knee cartilage than a standard radiograph (3). The cartilage in the knee joint is hyaline cartilage and is made up of Type II collagen fibers that cross link with type IX collagen fibers (5). The thinning of this cartilage matrix is just one sign of osteoarthritis in a knee, as we can also appreciate changes within the subchondral bone as well (5).
knee osteoarthritis with joint space narrowing

Image 1. Medial joint space narrowing

Subchondral sclerosis, osteophyte formation, and subchondral cyst formation can all be seen with osteoarthritis (5). Subchondral sclerosis is the brightening of bone on an x-ray that is due to the increased osteoblastic activity in bone (2). This failed healing mechanism of the subchondral bone microfractures leads to a decrease in the capacity of the bone to transmit load (6). Sclerosis is decreased in patients with osteoporosis (1).
knee osteoarthritis with subchondral sclerosis

Image 2. Subchondral sclerosis, severe joint space narrowing, osteophytes

Osteophytes are new bone formation at the edge of joint margins (2). Osteophytes form as a result of increased bone turnover in an osteoarthritic knee, leading to endochondral ossification at the joint margins (6). The later stages of this process involve subchondral cyst formation and altered joint alignment (6).
knee osteoarthritis with osteophyte

Image 3. Osteophyte of the medial femoral condyle with joint space narrowin.

Once the knee is evaluated, there are many grading scales that exist to categorize the severity of the osteoarthritis. The most common scales are the Kellgren-Lawrence, Ahlaback, and Brandt scale (4). A study published in the American Journal of Roentgenology looked at arthroscopic validation of the various grading scales of osteoarthritis in the tibiofemoral joint (4). What they found was that each grading scale had their own flaws (4). For example, the Kellgren-Lawrence grading scale which can be seen in Figure 4 relies on the presence of osteophytes for the first two grades (4). This can be misleading because according to the Kellgren-Lawrence scale, a knee with only joint space narrowing and no osteophyte formation would not meet criteria for osteoarthritis (4). This is in contrast to the Brandt scale that highlights joint space narrowing and not osteophyte formation (4).
kellgren lawrence scale of osteoarthritis

Figure 4. The kellgren-lawrence classification for knee osteoarthritis

Conclusion

Being able to recognize the signs of osteoarthritis on a radiograph is a necessary skill for a sports medicine physician. The incidence of knee OA is increasing in our population and diagnosing knee OA with a standard radiograph can be a cost saving measure. Being able to recognize joint space narrowing, subchondral sclerosis, and osteophyte formation will allow providers to make the correct diagnosis.

Case Answer

The correct answer is A. Subchondral sclerosis is found in radiographs with knee osteoarthritis. A Pelligrini Stieda sign is seen in injuries to the MCL. Symmetric joint space narrowing and erosions are found in Rheumatoid Arthritis.

References

Helms, C. (2020). Fundamentals of Skeletal Radiology. Philadelphia: Elsevier.
Andreas Adams, e. a. (2015). Grainger & Allison’s Diagnostic Radiology, Seventh Edition. Philadelphia: Elsevier.
Chan, W P, et al. “Osteoarthritis of the Knee: Comparison of Radiography, CT, and MR Imaging to Assess Extent and Severity.” American Journal of Roentgenology, vol. 157, no. 4, Oct. 1991, pp. 799–806, 10.2214/ajr.157.4.1892040.
Kijowski, Richard, et al. “Arthroscopic Validation of Radiographic Grading Scales of Osteoarthritis of the Tibiofemoral Joint.” AJR. American Journal of Roentgenology, vol. 187, no. 3, 1 Sept. 2006, pp. 794–799, pubmed.ncbi.nlm.nih.gov/16928947/, 10.2214/AJR.05.1123.
Cole, B. J., and C. D. Harner. “Degenerative Arthritis of the Knee in Active Patients: Evaluation and Management.” The Journal of the American Academy of Orthopaedic Surgeons, vol. 7, no. 6, 1 Nov. 1999, pp. 389–402, pubmed.ncbi.nlm.nih.gov/11505927/, 10.5435/00124635-199911000-00005.
Hannah Wilson, R. M. (2019). Knee. In M. M. Marc C. Hochberg MD, Rheumatology (pp. 690-703). Elsevier.
Buckland-Wright, Christopher. “Subchondral bone changes in hand and knee osteoarthritis detected by radiography.” Osteoarthritis and cartilage 12 (2004): 10-19.

Leave a Reply

Your email address will not be published. Required fields are marked *