What is the purpose of a squat? Is it to just squat, to lift maximal loads, to squat because it is a "great exercise", or do you want to challenge specific muscles? Another question worth asking is do you actually need to squat to achieve your exercise goal? If you're a competitive powerlifter then you probably do. If you're simply looking to improve your physique then maybe not.... maybe you can incorporate other exercises into your regime to achieve the same goals. If you have broken parts and are aren't able to elicit a contraction in one of the key muscles for executing a squat, then it might be worth addressing that before putting a bar on your back.
Something that is commonly missed with exercises is understanding how to bias them towards achieving a specific response or adaptation.
If you do decide to squat, then here's two (of many) ways you can possibly manipulate it to focus more on your exercise goal:
Knee Extensor Focus
If the goal is to develop strength in the knee extensor muscles (quadriceps group), then you might adapt your squat execution to increase the moment arm to the knee joint. This can be done by staying more upright and aiming to push the knees forward over the toes. By pushing the knees forward and staying more upright, we've increased the moment arm to the knee joint, with the aim of biasing more of the musculature of the knee extensors.
Hip Extensor Focus
If your goal is to challenge and create adaptation in your hip extensor musculature (glutes etc), then you might want to adapt your squat by increasing the moment arm to the hip joint. By pushing the hips back and increasing the distance between the line of force and the hip joint, we can create a greater challenge on the glutes and hamstrings relative to the quads. As you can see, simply squatting ass to grass might not end up achieving this, depending on the individuals body structure and capabilities.
So we have 2 squats with fairly different outcomes in terms of choreography and muscular challenge. Understanding how to manipulate exercise to challenge specific musculature can be extremely beneficial in both personal training and rehabilitation. This is something that is taught in more detail in the Exercise Mechanics Specialist Certification.