Knee pain is incredibly common, affecting around 19% of people in the UK at any given time [1]. It can affect your mood and impact your ability to work, exercise, get around and take part in your daily activities.
If your knee hurts when you bend and straighten it, this article’s for you. We’ll discuss possible causes when to see a doctor, and which treatments are available. All treatments discussed are offered by our highly-trained clinicians at Harley Street Specialist Hospital.
Why does my knee hurt when I bend and straighten it?
The knee is a complex joint, and many conditions can cause knee pain. Some of the most common causes of knee pain include:
- Arthritis
- Jumper’s knee
- Knee injury
- Runner’s knee
- Joint infection
- Bursitis
- Fibromyalgia
Let’s explore each of these potential causes in more detail.
Arthritis
There are many different types of arthritis, but osteoarthritis is the most common. In knee osteoarthritis (OA), the cartilage that cushions your knee breaks down – over time, this can cause the bones of your knee joint to rub together and break down too.
OA mainly affects older people, and over 73% of people with OA are more than 55 years old [2]. The knee is the most commonly affected joint, and 365 million people worldwide have knee OA [2].
Rheumatoid arthritis may also affect the knee – this is an autoimmune condition that causes your immune system to attack healthy tissue in your joints. Another type is psoriatic arthritis, a condition that occurs in people with psoriasis. Any type of arthritis may cause knee pain, swelling, and difficulty bending or straightening your knee.
Gout is also a type of arthritis. It causes sudden attacks of severe pain and swelling due to small crystals of uric acid that form in and around the knee joint.
1-2% of people in the UK are affected by gout; these are mainly men over 30 and post-menopausal women [3]. Symptoms come on rapidly and usually last 3-10 days before resolving. Unfortunately, most people experience further attacks of gout within the following year [3].

Jumper’s knee
The medical name for jumper’s knee is patellar tendonitis, which refers to inflammation of the patellar tendon. Your patellar tendon connects your kneecap (patella) to your shin bone (tibia).
Jumper’s knee is often a sports-related injury that occurs due to strain on the tendon when you repeatedly hit the ground. As the name suggests, it may be caused by frequent jumping on hard surfaces, such as when playing basketball.
Inflammation weakens your patella tendon and, if left untreated, can cause the tendon to tear. This condition is more common in men; research shows twice as many male non-elite athletes have jumper’s knee compared with female non-elite athletes [4].
Knee injury
Common knee injuries include sprains, strains, fractures, dislocations and ligament ruptures. These may be caused by falls or direct blows to the knee.
- Knee sprain – A knee sprain is when ligaments in your knee overstretch or partially tear. A common cause of knee sprains is twisting with a planted foot, for example, during an awkward landing when playing sports.
- Ligament rupture – A ligament rupture describes a complete tear of a knee ligament, so it splits into two pieces. This is more serious than a knee sprain and may require surgery.
- Muscle strain – A muscle strain occurs when one of the muscles attached to your knee is overstretched, and part of it tears. It may happen when playing sports, lifting something too heavy, or overusing the muscle.
- Fracture – A knee fracture is a broken bone in the knee. This is usually due to a fall or accident, although knee osteoporosis can cause the bones to become too brittle and fracture with minimal impact.
- Dislocation – Patellar dislocation accounts for 2-3% of all knee injuries [5]. This is when your kneecap slides out of place, usually to the side. It often happens due to a twisting injury or a direct blow to the inside of your knee.

Runner’s knee
Runner’s knee is the nickname for chondromalacia patella, a condition that often affects runners. It’s when the cartilage behind your kneecap breaks down, so your kneecap rubs against your thigh bone and causes pain.
Runner’s knee is often seen in people who do sports that put repetitive stress on the knee, such as running. People who are overweight are also at greater risk because of the increased pressure on their knee joints.
Joint infection
Viral, bacterial, or fungal infections can sometimes spread to your knee joint. This is called septic arthritis or a knee joint infection, and it is a serious medical condition.
The most obvious sign of a knee joint infection is sudden pain and swelling in your knee with no apparent injury. If you also experience a fever, chills, or feel generally unwell, seek medical attention immediately. You’ll need joint drainage and antibiotics if you have a joint infection.
Bursitis
Knee bursitis is when a small fluid-filled sac (bursa) near your knee becomes inflamed. There are several bursae in and around your knee joint; their role is to reduce friction between the tissues around your knee.
When these sacs become irritated, they produce too much fluid and can put pressure on the tissue around them. This can be painful and limit knee movement.
Fibromyalgia
Fibromyalgia is a chronic (long-lasting) condition that causes pain all over the body and extreme fatigue. It affects 2% of people in the UK, and 90% of people with fibromyalgia are women [6].
Fibromyalgia can affect your sleep and memory, and people with the condition are 20% more likely to also have depression or chronic anxiety [7]. Fibromyalgia pain is often described as a dull ache and can affect your joints, including the knees.
You may have constant or intermittent pain in one or both knees, which may be accompanied by swelling and stiffness. Knee pain is not a defining characteristic of fibromyalgia but may occur in patients with this condition, usually together with pain in other parts of the body.

When to see a doctor about knee pain
If your knee pain hasn’t improved within a few weeks, get advice from a doctor or physiotherapist. If you have any of the following symptoms, seek medical advice as soon as possible:
- You are not able to move your knee or take any weight on it
- You feel hot and shivery, are generally unwell, have a high temperature, and your knee is red or hot – these could be signs of infection
- Your knee is extremely painful
- Your knee has changed shape or is extremely swollen
- Your knee gives way, locks in one position or painfully clicks (pain-free clicking is normal)
Treatment for knee pain
Treatment options for knee pain include physiotherapy, steroid injections, regenerative treatments, and different types of knee surgery. Usually, surgery is considered only if you have serious knee damage or you have unsuccessfully tried other treatments.
Physiotherapy
Physiotherapy begins with a thorough assessment, including your medical history, symptom history and a physical assessment. During the physical assessment, your physiotherapist may look at your knee, hip, lower back and ankle.
They may feel around your knee for swelling or pain and will assess the movement of your knee as you bend and straighten it. They’ll also look at your gait (walking pattern) and see if you need any walking aids, like crutches.
Once they’ve assessed your symptoms, treatment will likely include an exercise programme to strengthen any weak muscles around your knee and improve your range of movement. They may also use passive treatments to relieve pain and improve movement, such as acupuncture, manual therapy, massage and electrotherapy.

Steroid injections
Steroid injections, also known as cortisone injections, can relieve pain and swelling in the knee. These injections usually contain corticosteroid medication to reduce pain and swelling over time, and a local anaesthetic to reduce pain immediately.
The number of steroid injections given in a year is usually limited due to potential side effects. There’s a risk of cartilage damage if you have too many steroid injections in a short timeframe [8].
The effect of a steroid injection usually wears off after a few weeks or months [9]. However, you may experience long-term relief if the period of reduced pain allows you to move more normally or do your physiotherapy exercises.
Regenerative treatments
We offer private regenerative treatments with cutting-edge technology that supports your body’s natural healing process. Regenerative medicine stimulates your cells to regenerate, restore and repair themselves. Our consultants specialise in:
- Platelet-Rich Plasma (PRP) therapy – With PRP, a blood sample is taken from your arm and then processed to create a concentrated sample of platelets. Platelets are important cells for healing and growth. This sample is reinjected into your knee to encourage repair and regeneration in the damaged area.
- Activated Mesenchymal Pericyte Plasma injections (AMPP®) – AMPP® combines Lipogems® with PRP. A blood sample is taken for PRP, and as part of Lipogems®, a sample of fat tissue is taken from your thigh or stomach. The blood and fat samples are processed to isolate regenerative cells and then injected into your knee joint.
- Bone Marrow Aspirate Concentrate (BMAC) – This process uses regenerative stem cells from your bone marrow. A bone marrow sample is taken from the centre of your pelvis bone and processed to create a concentrated sample of stem cells. This is injected into your knee joint, where the stem cells develop into the tissue your knee needs.

Surgery
In some cases, surgery is required to resolve your knee pain. You will usually require diagnostic imaging prior to treatment. Surgical knee pain treatments at Harley Street Specialist Hospital include:
- Knee replacement surgery – The surgeon makes a cut down the front of your knee to access the joint, then cuts away the damaged ends of your shin bone and thigh bone. Artificial parts are fitted over the ends of both bones. A partial knee replacement replaces only the arthritic part of the knee, and a total knee replacement replaces the entire knee joint.
- Knee arthroscopy – This is a type of keyhole knee surgery that can diagnose and treat knee problems. A knee arthroscopy lets the surgeon see your knee clearly and remove or adjust tissue as needed.
- Meniscus repair – Your meniscus is the cartilage that cushions your knee. If your cartilage is torn, your surgeon can often repair the cartilage while doing an arthroscopy.
- Knee ligament surgery – Repairing one of the four ligaments in your knee involves replacing it with a healthy piece of tendon. This tendon may be taken from your hamstring or patella tendon.
- Knee osteotomy – A knee osteotomy involves cutting the bones around your knee joint and reshaping them to change their alignment. This can help to relieve pain when taking weight through your knee and improve your knee function.
- Knee tendon surgery – Knee tendon surgery is often needed to treat a patellar tendon tear. During surgery, the torn tendon is reattached to your kneecap.
- Kneecap trochleoplasty – Kneecap trochleoplasty is performed to treat instability of your kneecap, particularly if you’ve had dislocations. A new groove is shaped for your patella to sit in, which helps it stay in place.
Treating knee pain at Harley Street Specialist Hospital
If your knee hurts when you bend and straighten it, it’s vital to get the correct diagnosis and effective treatment. Choosing private treatment at Harley Street Specialist Hospital ensures you get the care you need when you need it.
Our consultants, physiotherapists and radiographers will put you at ease every step of the way. Book an appointment today to start your journey to recovery.