According to the book "Joint Structure and Function: A Comprehensive Analysis," arthritis can lead to several symptoms that affect the rest of the body, mostly compensatory in nature. Back pain is likely the result of a domino effect from the individual favoring the non-affected knee.
Arthritis is often originally caused by poor walking biomechanics. Any pain in the affected knee can cause an individual to favor the less painful side, further exacerbating the poor biomechanics and leading to more pain. Favoring one side will cause poor posture and thus unusual strain on joints, muscles and tendons.
Effects on Joints
This favoring of one side could result in a twisting of the femur, and thus the hip. A twisting of the femur can put the hip joint out of alignment and place excessive stress on the lumbar spine. Pain likely begins in the lower back and then works its way up to the thoracic and even cervical spine.
Effects on Muscles
Any strain on a joint will result in excessive pulling on the tendons, which attach muscles to bones. This pulling on the tendons can lead to muscle aches and pains, especially near joints. According to the American College of Sports Medicine, poor biomechanics and arthritis can affect the rest of the body. One postural deficiency can throw the entire body out of alignment.