Define the following terms and discuss their clinical significance.

1. Ovoid articular surface

2. Sellar articular surface

3. Close pack position

4. Least pack position

5. Capsular pattern

1. An ovoid surface is one that is curved throughout its extent. An unmodified ovoid has an equal curvature everywhere as with a basketball or soccer ball, while a modified ovoid has unequal curves such as a football or rugby ball. The unmodified ovoid allows three degrees of freedom while the modified has no ability to spin arthrokinematically and has only two degrees of freedom. Examples of the unmodifed ovoid type of surface are found in the glenohumeral and hip joints and of the modified, the tibiofemoral, zygopophyseal, intercarpal etc.

2. A sellar surface is one that possesses both concave and convex curvatures such as a saddle. The unmodified sellar has its concavity perpendicular to its convexity while the concavoconvex relationship is not perpendicular in a modified sellar. The unmodified sellar has two degrees of freedom (no spin) while the modified has only one. Examples of the unmodified sellar are the 1st mcp and sternoclavicular. Examples of a modified sellar are the interphalangeal, the talocrual and ulnohumeral.

3. The close pack position is the position of maximum stability of the joint. It is the position of maximum nutrition provided the joint is moving in and out of the close pack. Therefore it must be mobilized early. It is the position in which articular stability can be tested. It is characterized by the following:

   • Maximum ligamentous and capsular tautness
   • Maximum joint surface congruity
   • Least ability to allow distraction or other joint play movements
   • Least articular volume
   • Least transarticular pressure

4. The least pack position is the opposite of the close pack. It is the position of least joint stability (the loose pack position is any position that is not the close pack). It is the position that the joint takes up when inflamed, when the joint is splinted, and the position used in the peripheral joints to test arthrokinematics. It is characterized by:

   • Minimal ligamentous and capuslar tightness
   • Least joint congruity
   • Maximal ability to allow joint play
   • Maximum joint volume
   • Maximum transarticular pressure in weight bearing

5. The capsular pattern is the restriction of motion observed when arthritis or arthrosis is present. It is used in the diagnosis of joint inflammation.