Joint Space Narrowing in the Knee

Where the gap is. On an X-ray, bone shows up white, while cartilage and the menisci do not show at all. The dark band between the end of the thigh bone and the top of the shin bone is the joint space. When that band looks thinner than expected, the report calls it joint space narrowing. It is read as a sign that the cartilage has thinned, or that a meniscus has worn or shifted out of the joint, because the gap is made up of both (Hunter et al., 2006).

The words that come with it. Reports describe where the narrowing is and how much there is. These are the terms you are most likely to see.

Kellgren-Lawrence grades. Some reports give an overall grade from 0 to 4. Grade 0 means no changes of osteoarthritis and grade 1 means doubtful changes. Grade 2 means definite bony spurs and possible narrowing, and is the usual threshold for calling it osteoarthritis on an X-ray. Grade 3 means definite narrowing with several spurs, and grade 4 means marked narrowing with changes in the shape of the bone ends (Kellgren and Lawrence, 1957).

The same knee can be worded differently. The severity words are not standardised, and agreement between readers varies (Riddle et al., 2013). A change in wording from one report to the next is not proof that the knee has changed.

The main guidelines agree on the first line of care at every X-ray grade: information about the condition, exercise, and weight management where weight is a factor (NICE, 2022; OARSI, 2019).

• Exercise. Exercise had a similar effect whatever the X-ray grade, and programmes that focused on the quadriceps and were supervised at least three times a week did better (Juhl et al., 2014). Loading exercise did not harm cartilage on MRI (Bricca et al., 2019). Pain can rise a little when you start. Across trials the average benefit is modest, and people with more pain at the start tend to gain more (Lawford et al., 2024; Holden et al., 2023).
• Even with severe changes. In a trial of people already eligible for knee replacement, a 12-week programme of exercise, education and diet support improved pain and function, and two in three of them had not had the operation two years later (Skou et al., 2015 and 2018).
• Weight. For people carrying extra weight, losing more than 5 percent of body weight can improve function, and around 10 percent or more tends to give larger benefits (Christensen et al., 2007; Messier et al., 2018).
• Medication and injections. Topical anti-inflammatories are a common next step, and a steroid injection can give short-term relief. These are arranged through your doctor (NICE, 2022).
• Not recommended. Arthroscopic “clean-out” surgery for arthritic knees (Siemieniuk et al., 2017), hyaluronic acid injections (NICE, 2022), and platelet-rich plasma or stem cell injections (OARSI, 2019).
• A surgical opinion. Worth asking for when symptoms substantially affect your quality of life and non-surgical care has not been enough. Your family doctor can arrange the referral.

I start with the knee, not the report. Where the pain sits and whether it matches the part of the joint the report names, how long the knee is stiff in the morning, whether it swells, gives way or locks, and what you have already tried. I also ask what you were told about the X-ray, because words like “bone on bone” shape what people feel safe doing.

On examination I look at alignment and walking, check for swelling and warmth, measure range of motion, and test quadriceps and hip strength, balance and single-leg control. I screen the hip and lower back, because both can send pain to the knee. I record a few measures I can repeat, such as a 30-second sit-to-stand and a timed walk, so you can see whether the plan is working.

The plan is built around strengthening, especially the quadriceps, with walking or cycling and balance work, progressed as the knee tolerates it. Joint mobilization and soft tissue work sit alongside the exercise where they make it easier. I explain what to expect when the knee flares, and I coordinate with your family doctor when medication, an injection or a surgical opinion is worth discussing.