An exercise routine incorporating a Pilates Ball maybe ideal for posture, strengthening the core and over time, creating noticeable and defined abdomen muscles, but it will do very little to reduce belly flab.
Belly flab, is the most difficult areas of the body to reduce; a controlled diet, less food and portion sizes alone will eventually reduce the belly flab but will take absolutely ages, even if a person stopped eating.
Incorporating a strenuous exercise routine, such as skipping (Jump rope) will drastically acheive the desired end results, although such an activity is probably not suitable for the OP.
The whole idea of an exercise programme in unison with a controlled diet is to speed up the desired end results; to see and feel a difference of appearance and level of fitness achieved, which encourages a participant to persevere..
Due to OPs joint pain and concerns in regard to abdominal scaring and healing; paced walking in combination with a healthy reduction of food and drink consumed, is by far the best solution.