Hi and welcome to XXXXXXX Thanks for the query.

It is practically not possible displacement of knee joint withour significant trauma since this is very stable joint with ligamentar and menisceal connections. unlike hip joint or shoulder which are more commonly subluxated or luxated.
In your case this sounds like arhritis,usually degenerative but also can be posttaumatic or rheumatoid. knee pain and "cracking" is commonly seen in these conditions. There can be inflammation of tendons, knee bursitis or cartilage damage. Rarely there can be damage of ligaments and meniscus. The first thing you should do is Xray exam which can verify the degree of artritis or joint stability. Muscles,tendons and ligaments should be checked by MRI because Xray isnt sensitive for it. Also, sometimes cracking can be normal because this happanes when there is sudden pressure on knee bursa and feeling of dispacement is just a subjective feeling.

If there is artritis it is recommended to change lifestyle habits such as losing weight, switching from running or jumping exercises to swimming or cycling, and minimizing activities that aggravate the condition, such as climbing stairs.

Exercises can help increase range of motion and flexibility as well as help strengthen the muscles in the leg. Physical therapy and exercise are often effective in reducing pain and improving function. Your physician or a physical therapist can help develop an individualized exercise program that meets your needs and lifestyle

Using supportive devices, such as a cane, wearing energy-absorbing shoes or inserts, or wearing a brace or knee sleeve can be helpful.
Other measures may include applications of heat or ice, water exercises, liniments or elastic bandages.

Next step are drugs and various of them are used.You should consult your gP about it. The last option is surgery,but just go step by step,do xray,ultrasound or MRI to verify the exact diagnosis.
Wish you good health. Regards

---