A surgical instrument system which comprises: a proximal base, a distal base, a support frame, a first arm having first and second ends, the first end of the first arm being connected to the support frame and the second end of the first arm being connected to the proximal base, the first arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, a second arm having first and second ends, the first end of the second arm being connected to the support frame and the second end of the first arm being connected to one of the bases, the second arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, a third arm having first and second ends, the first end of the third arm being connected to the support frame and the second end of the third arm being connected to the distal base, the third arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, and a tensioning mechanism (41, 46, 52) for increasing the tension in each tension member, characterised in that the system is for resecting a portion of a bone at a joint of a patient's limb with the proximal base sized and shaped to be mountable on the exterior of the patient's limb on the proximal side of the joint and the distal base sized and shaped to be mountable on the exterior of the patient's limb on the distal side of the joint, the system including: a first computer navigation tracker which is attached to the proximal base, a second computer navigation tracker which is configured for attachment to a patient's bone, a resection guide selectively mountable to the support frame, and a third computer navigation tracker mounting to the resection guide. a proximal base, a distal base, a support frame, a first arm having first and second ends, the first end of the first arm being connected to the support frame and the second end of the first arm being connected to the proximal base, the first arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, a second arm having first and second ends, the first end of the second arm being connected to the support frame and the second end of the first arm being connected to one of the bases, the second arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, a third arm having first and second ends, the first end of the third arm being connected to the support frame and the second end of the third arm being connected to the distal base, the third arm including a tension member having first and second ends and a plurality of ball and socket members slidably strung along the tension member to form a plurality of articulating linkages, and a tensioning mechanism (41, 46, 52) for increasing the tension in each tension member, characterised in that the system is for resecting a portion of a bone at a joint of a patient's limb with the proximal base sized and shaped to be mountable on the exterior of the patient's limb on the proximal side of the joint and the distal base sized and shaped to be mountable on the exterior of the patient's limb on the distal side of the joint, the system including: a first computer navigation tracker which is attached to the proximal base, a second computer navigation tracker which is configured for attachment to a patient's bone, a resection guide selectively mountable to the support frame, and a third computer navigation tracker mounting to the resection guide. a first computer navigation tracker which is attached to the proximal base, a second computer navigation tracker which is configured for attachment to a patient's bone, a resection guide selectively mountable to the support frame, and a third computer navigation tracker mounting to the resection guide.