What would happen in the market for knee replacement surgery if insurance companies started to cover a smaller portion of the cost of the surgery, and fewer doctors decide to enter the field of joint replacement surgery

Demand and supply will both decrease.

Explanation:

Once insurance companies starts covering only a smaller portion of the surgery cost, this will make people not want to even go through the surgery since insurance cannot pay for majority of the bill, thus demand will decrease. And if doctors for joint replacement surgery becomes fewer, this means there will not be enough doctors to carry out the procedure, and hence supply will decrease also.