Kinematics Team FAQ: Fusion of SI Joints (lower back pain)
August 18thFusion of SI joints
Your sacroiliac joints, or SI joints, sit where the base of your spine meets your pelvis. They transfer force between your upper body and legs as you walk.
Fusion means bone has formed across part or all of an SI joint. The medical term is ankylosis. This is not the same as a pelvis that simply feels stiff or “locked”. True fusion is a structural change usually seen on an X-ray, CT scan or MRI.
This can be related to ankylosing spondylitis, a form of axial spondyloarthritis affecting the spine and pelvis. Physiotherapy can’t separate the joints, but it can reduce pain and stiffness, improve strength and make movement in daily life easier.
Physiotherapy alone will not treat a fused joint. But your physiotherapist can assist you to make your daily movement more comfortable by reducing pain and stiffness and improving strength.
Where will I feel pain with fusion of SI joints?
You are likely to feel the pain deep in one or both buttocks, or it could be very low in the back. There are also instances where the pain is below the belt line at the back of the pelvis.
It is an ache that’s broader in area than a specific sharp spot. Pain levels do not always match the scan. Symptoms may also come from inflammation, nearby muscles, the hips or another part of the lower back.
What movements will I struggle to perform if I have fusion in my SI joints?
Movement may feel harder after rest. Getting out of bed, standing after a meeting or walking after a long drive can feel awkward, then ease after a few minutes.
Stairs, stepping into the car and other one-legged tasks may also be uncomfortable. Rehabilitation does not force the fused joint to move. It helps your hips, spine and surrounding muscles use the movement that remains.
Will I feel pain in any other regions of my body?
Discomfort may spread through the buttock, around the outside of the hip, towards the groin or into the upper thigh. Stiffness can also appear higher in the lower back as nearby areas take on extra work.
If you experience any eye inflammation, bowel problems, numbness, or leg weakness, seek urgent assessment.
How does a SI joint fusion happen?
True fusion does not develop overnight. It’s a gradual new bone growth across the joint because of continuous inflammation or repeated injury. Eventually, this starts to limit your movement.
One possible cause is ankylosing spondylitis which begins around the SI joints. This causes lower back pain that is worse in the morning or after a periodical rest, then eases with movement.
Previous injury or age can also impact joint health. Pain around the SI region does not automatically mean fusion, so imaging and medical review may be needed.
What things should I try initially once I’ve fusion of SI joints?
It is best to start with gentle movements. Walking, stretches that are comfortable, or mild exercises in between desk work can be useful.
Warm showers are also helpful to ease morning stiffness. The heat helps loosen tension of the lower back and buttocks muscles.
Tell your GP if pain is worse after rest, wakes you in the second half of the night or clearly improves with activity. A rheumatology review may be appropriate if inflammation is suspected.
What things should I try to avoid in the initial stages of SI joints fusion?
Avoid repeatedly twisting or forcing the pelvis to “unlock” it. You cannot stretch out joints that are fused. Aggravating the area may even cause irritation and worsen the symptom.
The better approach is to reduce loads that are causing discomfort, then work with your physiotherapist to rebuild strength and mobility gradually. You also need to watch out for other issues in the hips, muscle irritation, or typical lumbar pain as these can feel similar but probably not always coming from your fused joint episode.
What type of exercises or movements do I need to be stronger at to rehab SI joints?
Hip strength helps control the pelvis during walking, stairs and single-leg balance. Bridges, sit-to-stands, step-ups and resistance-band exercises are common options.
Your trunk muscles transfer force between the upper and lower body, so rehabilitation may include modified planks, controlled carries or lifting practice. Mobility work may focus on the hips, upper back and parts of the lower spine that still move comfortably.
For axial spondyloarthritis, programmes may also include posture, breathing, spinal mobility, strengthening and aerobic exercise.
What other considerations contribute to fusion of SI joints?
Management depends on the cause. If there is inflammation, your physiotherapy session is typically combined with a medical treatment.
Lifestyle can affect how stiff or sore you feel. Sleep, stress, fitness, work demands, and training load contributes to the level of discomfort.
Your physiotherapist will ask about how you move in your daily routine, especially movements concerning your lower back and hip. Assessment will also include other symptoms that are time specific – how is your pain during nights and during daytime when you are moving more.
What treatments can help relieve some pain or fast track my SI joints rehabilitation?
Massage, manipulation and exercise cannot reverse a fused SI joint. Treatment aims to settle pain, maintain movement elsewhere and build strength for daily activities.
Your treatment plan may be a combination of exercises, load management, and optimising your movements that are integrated in your daily activities. Hands-on treatment may also be given to target stiffness which supports your active rehabilitation.
Medications or a specialist care may also be supplemented in cases where axial spondyloarthritis is diagnosed.
What are the expected timeframes of rehabilitating SI joints?
SI joint fusion is permanent. Rehabilitation does not cure it but rather helps with function, minimising symptoms, and supporting mobility so you are more confident in moving.
Improvements on simple movements like walking may be noticeable sooner, while movements that require more load will improve gradually.
Progress is not always steady. Inflammation, poor sleep, work demands and sudden increases in activity can cause setbacks. A good programme adjusts when needed and keeps building capacity once symptoms settle.
Book a physiotherapy assessment at Kinematics in Richmond for any lower back or buttock pain. Our team will help identify what is contributing to your pain and guide your return to activities through a personalised rehabilitation programme.
This article is for general education and is not a substitute for personalised medical advice.